Knock, knock!

Who’s There?

Theodore.

Theodore who?

Theodore is stuck and it won’t open!

Explanation:  Yesterday, teddy bears.  Today, Theodore: thee-o door.  The last part of the name “Theodore” sounds like “door.”  If you say the whole name fast it can almost sound like “thee door” or “the door.”  This joke is funny because it is not Theodore who is knocking at the door but someone who says the door is stuck.