Q: Why do scientists think humming birds hum?
A: Because they can’t remember the words!

Explanation:  “To hum” can be to sing, “la, la, la” when you don’t know the words to a song or keep your lips together and make the notes of a song. This joke is funny because hummingbirds don’t really hum; they chirp. They’re birds!  I wonder why they can’t remember the words?

Here is a scene where Ingrid Bergman hums a few bars.