here is a nice one I used the other day, its quite famous and you can find the solution all over the web, its good because there is a very obvious solution and there is a very hard solution and its good to see how someone thinks.
So here goes see if you can solve it without cheating.

There are two trains facing each other on a 200 mile length of track, sitting on the front of the first train is a super fly which can fly at 75 miles per hour.
Both trains set off heading for each other at the same time travelling at 50 miles per hour. The fly flies from the first train to the front of the second train, meets the second train, turns round flies back to the first and so on.
The question is how far does the fly fly before the two trains collide?
+50 points for the obvious solution
-5000000 points for the difficult solution.