Did you hear about the man who swallowed his watch?
He went to the doctor hoping he could give him something to help pass the time.
Listen up, dummy. The joke is that the dude swallowed a watch, right? Watches tell time. So, the punchline is a play on words. "Pass the time" can mean two things: to literally excrete the watch, or to make time go by faster. It's funny 'cause it's unexpected and kinda stupid, just like you!