I told my almost-seven-year old that she is "smart as a whip."
She said: "Whips aren't very smart, Dad. Are you trying to say that I am not very smart?"
I assured her that was not the case. She then told me that I could have at least said that she was "smart as a dolphin" which, although not as smart as human beings, are at least well known for their intelligence (unlike whips).