 A second class of determinism-breaking models can be constructed on the basis of collision phenomena. The first problem is that of multiple-particle collisions for which Newtonian particle mechanics simply does not have a prescription for what happens. (Consider three identical point-particles approaching each other at 120 degree angles and colliding simultaneously. That they bounce back along their approach trajectories is possible; but it is equally possible for them to bounce in other directions (again with 120 degree angles between their paths), so long as momentum conservation is respected.)