Some fairly common expressions, although well-understood, don't seem to make much sense. One of them is as smart as a whip.
She's as smart as a whip,
And Cas is also quite hip.
One that is not hard to understand is stubborn as a mule.
He's as stubborn as a Missouri mule,
And you'd never know Ron has been to school.
Missouri is, of course, known for its mules. One well-known story is about the farmer that was seen hitting his mule with a 2x4. A city slicker happened to see a farmer hitting his mule with a big stick. He asked the farmer why he was doing that. The farmer responded, "Well, first you have to get his attention."