Bill Cosby gives bizarre sex talk on 'Tonight Show'

Bill Cosby can explain sex in your 70s, but he cannot "understand the intelligence of a moth."

That sentence doesn't even begin to describe the 77-year-old comedian's bizarre antics on Monday night's "Tonight Show" with host Jimmy Fallon.

WATCH: Jimmy Fallon Pelts a Ball at Julia Roberts' Face!

For example, Cosby greeted the audience by saying, "We're here! We're here on the Fallon show and we're waiting for the gas pedal and the clutch and the brake... Too many people have hit the deer, and the deer always win. But the ones I don't understand -- I don't understand the intelligence of a moth."

Viewers didn't even have time to let that strange sentence set in before the funnyman was talking about having sex with his 70-year-old wife for his birthday. "The most frightening things that you can hear from your wife on your birthday when you are carrying on to hear her say, 'Keep going,'" he proclaimed.

PHOTOS: Must-See Celebrity Selfies

He then began to repeat himself and instructed Fallon on what to do when he said, "Keep going." Like the rest of the audience, Fallon didn't seem to get it. Cosby then got the floor and crawled around as a frantic Fallon pleaded with him to get up.

Check out the clip below to see the strange moment for yourself.