A Super Toy

Too bad, that wasn't correct.
It is true: in reality, collisions are never ideal processes. A certain amount of the energy is always transformed into warmth which means that there is less energy left for the movement. Yet, the result of the little experiment is still different from what you have described.

You haven't found the right anser, yet, but another helpful hint can lead you onto the right track. Don't let it discourage you! Use another helpful hint and rethink.