Q: What musical instrument is found in the bathroom?
A: A tuba toothpaste.

Explanation: A ‘tuba‘ is a large musical instrument, a brass instrument, that plays very low notes when you blow into it.

A tube of toothpaste is, well, toothpaste that comes in a tube and it is usually kept in the bathroom.

If you say “tube of toothpaste” fast it sounds like “tuba toothpaste” and that is what makes this joke funny.

Here is an old commercial for Ipana Toothpaste with Bucky Beaver: