I was just messing around the other night and I landed on 20Q.net. I thought "There's no way this thing can possibly guess my item." So I thought of some of my favorite things. A Scrabble gameboard (which the game correctly guessed after 14 questions), a hockey puck (which the game correctly guessed after 17 questions), a bottle of beer (which it correctly guessed after 10 questions). I was a little freaked out, so I started to think of random things. An emu, garlic, a washing machine, a zippo lighter, dental floss, dryer sheets, a thesaurus, a flashlight. All of these were correctly guessed by the game. I really don't know how it does it. I was really frustrated. I started to think of more obscure or specific things, like jicama and a staple remover, which were also correctly guessed within 30 tries. I only stumped it a few times out of MANY. I think it stops trying after 30, and asks you to tell it. Then it informs you that you were wrong to answer "yes" to certain questions or "no" to certain others.
If you have a few minutes of free time and feel like becoming really frustrated and amused, check it out.