- Marijuana will be sold like alcohol in Colorado, to those age 21 and older
- Residents can buy up to an ounce, and out-of-staters can buy a quarter ounce
- Colorado becomes the first state in the nation to open recreational pot stores
- It's also the first in the world where pot will be regulated from seed to sale
Denver (CNN) -- Colorado will begin allowing recreational marijuana sales on January 1 to anyone age 21 or over.
Residents will be able to buy marijuana like alcohol -- except the cannabis purchase is limited to an ounce, which is substantial enough to cost about $200 or more.
It's a big moment: Colorado will become the first state in the nation to open recreational pot stores and become the first place in the world where marijuana will be regulated from seed to sale. Pot, by the way, is the third most popular recreational drug in America, after alcohol and tobacco, according to the marijuana reform group NORML.
Here are 10 things to know about what will be a closely watched landmark law.
There appears to be a shift in the United States in favor of relaxing marijuana laws, a topic that has dipped in and out of the national conversation for decades. Public perceptions about pot have come a long way, from the dire warnings of "Reefer Madness" to growing acceptance of medical marijuana. 
Rick Ridder, left, high-fives Christian Sederberg at a victory party in downtown Denver. Advocates for Proposition AA celebrated on Tuesday, November 5, after The Associated Press and the Denver Post called the vote in their favor. Proposition AA would impose a pair of taxes on legal marijuana sales. 
Members of a crowd numbering tens of thousands smoke and listen to live music at the Denver 420 Rally on April 20. Annual festivals celebrating marijuana are held around the world on April 20, a counterculture holiday. 
A man smokes a joint during the official opening night of Club 64, a marijuana social club in Denver, on New Year's Eve 2012. Voters in Colorado and Washington state passed referendums to legalize recreational marijuana on November 6, 2012. 
People light up near the Space Needle in Seattle after the law legalizing the recreational use of marijuana went into effect in Washington on December 6, 2012. 
Nutrient products are placed on shelves in the weGrow marijuana cultivation supply store during its grand opening on March 30, 2012, in Washington, D.C. The store is a one-stop-shop for supplies and training to grow plants indoors, except for the actual marijuana plants or seeds. Legislation was enacted in 2010 authorizing the establishment of regulated medical marijuana dispensaries in the nation's capital. 
Marijuana activist Steve DeAngelo wears a "Yes on Prop 19" button as he speaks during a news conference in Oakland, California, on October 12, 2010, to bring attention to the state measure to legalize marijuana for recreational purposes in California. Voters rejected the proposal. 
Sonja Gibbins walks through her growing warehouse in Fort Collins, Colorado, on April 19, 2010. Since the state approved medical marijuana in 2000, Colorado has seen a boom in marijuana dispensaries, trade shows and related businesses. So far 20 states and the District of Columbia have made smoking marijuana for medical purposes legal. 
A patient prepares to smoke at home in Portland, Maine, on October 22, 2009, a decade after the state approved a medical marijuana referendum. 
Coffeeshop Blue Sky worker Jon Sarro, left, shows a customer different strains of medical marijuana on July 22, 2009, in Oakland, California. Voters in the city approved a measure during a vote-by-mail special election for a new tax on sales of medicinal marijuana at cannabis dispensaries. 
Medicinal marijuana patient Angel Raich wipes her eyes during a press conference on March 14, 2007, in Oakland, California. The 9th circuit U.S. Court of Appeals in San Francisco ruled that 41-year-old Raich, who used medicinal marijuana to curb pain from a brain tumor as well as other ailments, did not have the legal right to claim medical necessity to avoid the possibility of prosecution under federal drug laws. 
Different varieties of medical marijuana are seen at the Alternative Herbal Health Services cannabis dispensary in San Francisco on April 24, 2006. The Food and Drug Administration issued a controversial statement a week earlier rejecting the use of medical marijuana, declaring that there is no scientific evidence supporting use of the drug for medical treatment. 
People in New York gather for a pro-cannabis rally on May 4, 2002. That same day, almost 200 similar events took place around the world to advocate for marijuana legalization. It was dubbed the "Million Marijuana March." 
Dennis Peron takes notes during a phone interview while Gary Johnson lights up at the Proposition 215 headquarters in San Francisco on October 11, 1996. The ballot measure was approved when voters went to the polls in November, allowing medical marijuana in California. 
A television ad aired in 1996 by Republican presidential candidate Bob Dole's campaign included footage from a 1992 MTV interview of a laughing President Clinton saying he would inhale marijuana if given the chance to relive his college days. 
President George H. Bush holds up a copy of the National Drug Control Strategy during a meeting in the Oval Office on September 5, 1989. In a televised address to the nation, Bush asked Americans to join the war on drugs. 
Robert Randall smokes marijuana that was prescribed to treat his glaucoma in 1988. He became the first legal medical marijuana patient in modern America after winning a landmark case in 1976. 
First lady Nancy Reagan participates in a drug education class at Island Park Elementary School on Mercer Island, Washington, on February 14, 1984. She later recalled, "A little girl raised her hand and said, 'Mrs. Reagan, what do you do if somebody offers you drugs?' And I said, 'Well, you just say no.' And there it was born." She became known for her involvement in the "Just Say No" campaign. 
President Jimmy Carter, with his special assistant for health issues, Dr. Peter Bourne, beside him, talks to reporters at the White House about his drug abuse control message to Congress on August 2, 1977. Among other things, he called for the elimination of all federal criminal penalties for the possession of up to one ounce of marijuana. 
Panel members of the National Commission on Marijuana and Drug Abuse attend a hearing In Denver on January 10, 1972. From left, Dr. J. Thomas Ungerleider, psychiatrist; Michael R. Sonnenreich, commission executive director; Raymond P. Shafer, commission chairman; Mitchell Ware, Chicago attorney; Charles O. Galvin, Dallas law school dean. The commission's findings favored ending marijuana prohibition and adopting other methods to discourage use, but the Nixon administration refused to implement its recommendations. 
Protesters wade in the Reflecting Pool at the National Mall in Washington during the "Honor America Day Smoke-In" thrown by marijuana activists in response to the official "Honor America Day" rally organized by President Nixon supporters at the Lincoln Memorial on July 4, 1970. 
Marijuana reform was the Life magazine cover story in October 1969. The banner read: "At least 12 million Americans have now tried it. Are penalties too severe? Should it be legalized?" 
Police dogs trained to smell out hidden marijuana examine U.S. soldiers' luggage at the airport during the Vietnam War in 1969. Drug use was widespread during the war. 
People share a joint during a 1969 concert in Portland, Oregon. In 1973, Oregon became the first state to decriminalize cannabis. 
Marijuana use became more widespread in the 1960s, reflecting the rising counterculture movement. 
Research scientist Dr. Reese T. Jones, right, adjusts the electrodes monitoring a volunteer's brain response to sound during an experiment in 1969 that used a controlled dosage of marijuana. The tests were conducted at the Langley Porter Institute at the University of California, San Francisco. 
U.S. Customs agents track the nationwide marijuana market during Operation Intercept, an anti-drug measure announced by President Nixon in 1969. The initiative intended to keep Mexican marijuana from entering the United States. 
Members of the Grateful Dead talk with reporters from their home in San Francisco on October 5, 1967. The band was protesting being arrested for marijuana possession. 
A woman buys ready-rolled marijuana cigarettes from a dealer at her door circa 1955. 
Even after Congress cracked down on marijuana in 1937, farmers were encouraged to grow the crop for rope, sails and parachutes during World War II. The "Hemp for Victory" film was released in 1942 by the U.S. Department of Agriculture. 
Marijuana cigarettes are hidden in a book circa 1940. Congress passed the Marijuana Tax Act in 1937, effectively criminalizing the drug. 
A poster advertises the 1936 scare film "Reefer Madness," which described marijuana as a "violent narcotic" that first renders "sudden, violent, uncontrollable laughter" on its users before "dangerous hallucinations" and then "acts of shocking violence ... ending often in incurable insanity." 
Harry Anslinger was named commissioner of the Federal Bureau of Narcotics when it was established in 1930. While arguing for marijuana prohibition, he played on Americans' fear of crime and foreigners. He spun tales of people driven to insanity or murder after ingesting the drug and spoke of the 2 to 3 tons of grass being produced in Mexico. "This, the Mexicans make into cigarettes, which they sell at two for 25 cents, mostly to white high school students," Anslinger told Congress. 
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Photos: History of marijuana in America How can this be?
Colorado pot sales to begin in 2014 Voters wanted this. And the law is now in the Colorado constitution after 65% of voters said yes to legalizing recreational marijuana.
Colorado readies for recreational pot Colorado wasn't the only state to OK this in November 2012. Voters in Washington also said yes, but that state won't open marijuana retail outlets until later in 2014.
Why?
There are the usual "legalize it" arguments about how pot is less dangerous than alcohol or tobacco and how legalization would save taxpayers $10 billion yearly on enforcing the prohibition.
Then there's the reality we all know: There will be a tax bonanza to public treasuries.
Retail weed will have a 25% state tax -- plus the usual state sales tax of 2.9% -- making recreational pot one of the most heavily taxed consumer products in Colorado. Some communities are adding even more taxes to the product.
The additional revenue will initially amount to $67 million a year, with $27.5 million of it designated to build schools, state tax officials say.
So why bother with separate medical marijuana?
Because buyers of medical pot won't face the additional taxes.
Medicinal weed in Colorado still requires a physician's recommendation, and the dispensaries will be separate outlets from the recreational pot retailers.
How much recreational weed can I buy?
If you are 21 or older, you can buy up to an ounce at a licensed store, as long as you have a Colorado ID. People from outside Colorado can buy a quarter ounce.
Only a handful of stores, however, are expected to open on January 1, and Denver will be home to many of them, according to the Denver Post and the weekly Denver Westword. In fact, there are concerns that supplies will be sold out on the first day, with so few stores having passed the lengthy licensing process so far. About 160 retailers are still seeking licenses statewide.
Users can also share an ounce of cannabis with a friend as long as no money is exchanged.
Where can I light up?
You won't be allowed to smoke pot in public and, in fact, can't even smoke in the pot shop or other establishments governed by the state's Clean Indoor Air Act.
That leaves the smoking to private properties, with the owner's permission.
Communities and counties can still choose not to allow recreational marijuana stores in their local jurisdictions, and a good many towns have, such as Colorado Springs and Greeley.
Meanwhile, ski resorts are concerned about scofflaws lighting up while on the slopes, with smoke intruding on family settings.
Can I grow my own?
Yes, you can grow up to six plants in your home, but the pot patch must be enclosed and locked.
Can the underage get busted for pot?
Yes, it's illegal to possess and use marijuana if you're under 21, but the city of Denver this month decriminalized pot for people between ages 18 and 21. The city would keep the fines -- but remove the jail time -- for being caught with an ounce or less. The potential jail time had been up to a year.
Youths under age 18 could be sent to a juvenile assessment center, instead of jail. The measure ensures kids "don't have to live into adulthood with mistakes they might have made when they were 19," Councilman Albus Brooks said in a Denver Post article.
What about DUI?
A motorist in Colorado can be ticketed for impaired driving if his or her blood shows more than 5 nanograms of active THC, the active constituent of marijuana, NORML says on its website.
Some users will fall below that level three hours after consuming pot, but "some people will still be well above 5 ng," NORML says. "Do recognize that the effects of alcohol and marijuana together may be more than the sum of their parts."
Some analysts describe impairment as a guessing game, depending on the person.
"Is Colorado's marijuana DUI rule flawless? Far from it. But as the state's policymakers have come to realize, the world's first legal pot rules aren't going to be perfect. They just have to be good enough. Good enough to keep the feds away, good enough to keep marijuana stakeholders happy, good enough to keep Coloradans from worrying they've made a horrible mistake," University of Denver law professor Sam Kamin and writer Joel Warner wrote in Slate this month.
And what about the feds?
It's always been a murky relationship between the feds and those states with laws authorizing medical -- and now recreational -- marijuana. Federal law says the drug's possession, manufacture, and sale is illegal, punishable by up to life in prison, and its mass cultivation is a sensitive subject among growers, experts say.
But in August, the U.S. Justice Department said it won't challenge Colorado or other states with laws legalizing recreational marijuana. Instead, federal officials will focus on serious trafficking and keeping the drug away from children.
Does this confuse you?
It should, one legal analyst says.
"They should be confused," attorney Alan Dershowitz said. "The federal government still takes the position technically that you're violating federal law if you're complying with the state law. But the Obama administration, I believe, has recently has taken a turn on its approach to drug enforcement."
Can I giggle?
Let the jokes and puns begin -- stoned or not.
Even Colorado NORML is being cheeky about it, posting online a list of what's allowable under the new recreational pot law.
It's called "Doobie-DOs."
CNN's Miguel Marquez, Ana Cabrera and Sara Weisfeldt contributed to this report.
