Two people in a group each think of an object in their head. They count down and say the objects at the same time. Then, two other people in the group try to think of objects that are "half-way between" the two that were just said. This repeats until the two people say the same object.

You can't say an object that's already been said, but it's fine for the same person to go twice if no one else can think of anything.