What musical instrument is found in the bathroom?
Why this joke works
Music by the Bathroom Sink
Tuba toothpaste sounds like tube of toothpaste when of is spoken lightly. The question asks for a musical instrument, and the reply disguises a familiar bathroom object as one. Join the words smoothly rather than pausing after tuba. The toothpaste needs to arrive with the instrument name so listeners can hear the ordinary everyday phrase hiding underneath the unexpected answer.
A closer look
Moving the boundary between words
The trick changes word boundaries rather than simply replacing a noun. Tube of becomes tuba, leaving toothpaste to anchor the answer in the bathroom. Written text makes the instrument spelling visible, while spoken delivery supplies the compressed phrase. A listener who reads of with a strong separate syllable may need to hear the whole answer once at conversational speed.
Fact behind the joke
The low voice of the brass family
The tuba provides the lowest-pitched voice in the standard brass family described by Yamaha's instrument guide. It is a large wind instrument, not a bathroom accessory. That contrast between a substantial musical instrument and a small tube at the sink makes the imagined object especially unlikely.