Interesting Thought Experiments

What are some interesting though experiments you have? Philosophical, scientific, or even simply fun ones. Some rather boring ones can be found here: http://plato.stanford.edu/entries/thought-experiment/

I think one that's fun to think about is this:
Imagine that it turns out you're actually in a box. This box is about the size of a teacher's desk or so, but it simulates what you perceive is your world. Only you are in this box. What would that say about the distinction between Illusion and reality? Just interesting to think about when you get in the right place.

Any others?