A tidy little character study in four lines: good when good, horrid when not. Often credited to Longfellow, though nobody is entirely sure. The stark before-and-after makes it satisfying to say aloud, with a wag of the finger saved for the last word.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
There was a little girl who had a little curl
Right in the middle of her forehead;
When she was good, she was very, very good,
And when she was bad she was horrid.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition