What was the last thing on the minds of the 9/11 jumpers?
...
...
Their knees.
*Ba dum tss*
Listen up, buttercup. The joke is a dark twist on the 9/11 tragedy. Get it? The punchline is that the jumpers' knees were the last thing on their minds 'cause, like, they were falling to their deaths. I bet you think that's deep, huh? You probably think that show "The Big Bang Theory" is edgy too.