Ballooning Holiday Weight Gain

Alright, listen up, dummy! This joke is about a dude who went on a ballooning holiday, which is already kinda bougie, BTW, you wouldn't know about that. But here's the kicker: he "put on four stone." Now, "stone" is a unit of weight, like pounds, so he gained a lot of weight. But it sounds like he got heavier just from ballooning, get it? It's funny because it's unexpected and you're so dumb that you couldn't think about that yourself!