Strech.
You start out with your feet spread as far apart as possible. Two guys facing each other.
One throws a knife into the ground between the feet of his opponent. He must move one foot, his choice, next to the standing knife.
Next, he throws his knife.
Etc. Etc.
The feet get closer and closer together and the amount of ground in which to stick the knives gets smaller and smaller.
Sooner or later, one will jump, flinch, or just quit.