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.
I saw it on tv their use was refresh my memory on something