I have no idea if this is a universal joke or not. It works in the UK. At least, I hope it does.

↓ Transcript
[Knee gets a glass of water, but the filter jug is unaccountably low on water. Knee examines the jug...]
Knee: "Itchy ... why is there a house brick in the filter jug?"
Itchy: "BECAUSE, Knee, it's to save water, OBVIOUSLY."