Pick someone at random who's signed on.
Say "im a rock".
Keep on doing that, varying as needed (e.g. "are u a rock?" "why arent you a rock?")
See how long you can keep it going for!
Warning: the above may damage your sanity, the sanity of whoever your victim is and possibly your reputation (or at least make people wonder about you). But then again, that's the whole point of this!