I learned one in elementary school: each line is one note higher on the scale., then the "Meows" walk back down the scale.

I know a little pussy,
Her coat is silver gray.
She lives down in the meadow
Not very far away [or was it, "She'll never run away"?]
Although she is a pussy
She'll never be a cat
For she's a pussywillow
Now, what do you think of that?
Meow, meow,meow, meow, meow, meow, meow"
SCAT!