The Magazines Clause:
If you're ever talking about press and the law, just throw into the conversation, "Oh, that's covered by the Magazines Clause of the Constitution." Then watch them squirm until you admit that this Clause doesn't quite mean what you at first suggested it did. It's also a nifty example to illustrate noscitur a sociis.
Thanks to Stanford lawprof Pam Karlan for the pointer.