20 questions against a computer.
Found via kottke.org.

That’s amazing! The artificial intelligence system gets better and better anytime someone plays with it: it learns from his opponents.
The goal is (for the computer) to guess what you are thinking in 20 questions, it guessed right 7 out of 10 with me, it failed to “recognize” I was thinking a motorbike and said “engine - internal combustion”.
If it fails, usually, it gets close to your object.

Try it!