You have a bunch of ants on a meter stick, each walking 1cm/s in some direction. If an ant hits the end of the stick, it falls off. If two ants collide, they both reverse direction.
Walking Ants Puzzle earlier essentially asked: Given the starting positions and directions of all the ants, how long until the last ant falls off?
The new problem is :
Given the starting positions and directions of all the ants, which ant(s) are the last to fall off?
Disclaimer: I do not have the solution to the problem. It just looks like an interesting problem to solve.
( Readers Please ignore: Technorati claim: 6QVZ8YSY6XSD )
Update (24 June 2014):
Solution: Posted by Strilanc and me in comments!