I saw a variation of this idea once with a space game. IIRC you control a ship and asteroids attack you. You shoot the asteroids by clicking on them.

The first trick: the asteroids are actually (in)visible depending on whether you had visited a specific website, same as here. If you clicked on an asteroid that's because you could see it, and now you've revealed that you've visited that website before.

Second trick: these asteroids don't actually hit you - they pass close by (close enough that you want to shoot them) but they don't actually aim at you (to keep you playing as long as possible).