Comedy Pen
You pretend to write with it, and it does not write. So you scribble with it on a table cloth or someone's shirt to start the ink flowing. The pen leaves a clear black mark. Just when the owner of the shirt or tablecloth is about to blow a fuse or bawl you out, you blow on the ink mark, and it vanishes!
No special chemicals or liquids. Use the same pen over ad over again. Nothing to refill or replace.
