Q: What happened to the mouse when he fell into the bathtub?
A: He came out squeaky clean!
Explanation: Just as a cow says “moo” and a pig says “oink,” mice squeak. If you are interested in hearing an hour of mice squeaking you can listen here (although I am not sure why you would want to).
Squeaky clean is an informal phrase meaning that something is really, really clean. A window that you just washed could be called squeaky clean. Squeaky clean can also refer to a person’s character–the senator was squeaky clean: he never lied, misled or cheated anyone.
This joke is funny because it plays with the word squeaky, meaning the sound of a mouse and completely clean.
Here is Tom and Jerry getting things squeaky clean–