Comedy legend and retired pot smoker David Letterman - who turns 69 today - is probably best known for putting guests on the spot by springing unexpected questions on them. "That's what I do: I make people feel uncomfortable," he once told Justin Bieber after a tense one-on-one.

And marijuana was one of his go-to topics for generating awkward laughs. Since he already owns the Top 10 list, we've put together a sampling of Letterman's top 5 marijuana moments from his 22-year tenure as the host of The Late Show (1993-2015).

1. Oprah and Dave open up about the munchies

"What do you think about smokin' weed?" Letterman asked Winfrey in abrupt fashion during one of his last episodes. She hadn't smoked in years, she said. Dave said the same, adding that he decided to quit after eating two pints of ice cream back-to-back. "For me, it was a sleeve of Oreos," Winfrey replied. Check out the full segment:

2. Bill O'Reilly's compatibility test

In October 2014, Letterman sat down with a list of questions to test his compatibility with political commentator Bill O'Reilly. Baseball or football? Lakes or rivers? Cake or pie? were some of the questions leading to the climax - weed or booze? Check out O'Reilly's response.

3. Top Ten cannabis courses

When Clover Leaf University - America's first marijuana-focused institution for higher learning - announced its opening in 2014, Dave couldn't resist putting together a Top 10 cannabis course-load, which includes Intermediate Cheech and Advanced Chong.

4. Seth Rogen endorses B.C. bud

"Are you loaded, uh, weeded, uh stoned right now?" Letterman stammered early in a 2012 interview with Seth Rogen.

"Not very, no," Rogen quipped. "I'm never really at zero....Just because there's a cumulative effect, so it's probably in there somewhere."

Letterman continued quizzing Rogen on the potency of today's cannabis, how to buy it in legal states and where the best marijuana's grown. Rogen's answer? His hometown of Vancouver, British Columbia.

5. Madonna puts Dave on the pot seat

There wasn't much Letterman and Madonna didn't discuss in their infamous, profanity-laden interview from 1994. This time, Madonna turns the table on Letterman, and asks the question herself. Toward the end of the gruelling verbal skirmishes between the pop star and host, Madonna asked Letterman, "Have your ever smoked endo?" That slang word for marijuana came up again later when Madonna helped Dave prepare a to-do list

h/t Rolling Stone