Stephen Hawking on Nitrous

Listen up, buttercup. The joke is about picturing Stephen Hawking, a genius dude who couldn't move much, getting a load of nitrous oxide—laughing gas—which makes people act all silly. It's funny 'cause it's unexpected and kinda wrong to imagine him like that. Since you are reading my explanation, I can tell that you are a silly goose, just like Stephen Hawking when he is on nitrous.