When two free neutrons collide at high speed do they ricochet off one another like billiard balls? If so, what is the force that they exert on one another in order to change their velocities? It clearly isn't gravitational or electromagnetic. The only other fundamental forces are the strong and weak nuclear forces. Is it one of those? If not, is it some other principle (eg Pauli exclusion principle) that somehow circumvents the need for a force to explain the ricochet?