Screw Loose

Alright listen up, buttercup. The joke is that Stephen Hawking fell over because he had a "screw loose." It's a play on words because "having a screw loose" means someone is not all there in the head or is a bit crazy. But in this case, it's a literal screw, like one that holds a machine together. You probably still don't get it and that's not surprising coming from someone who doesn't know how to tie their shoes. Sad!