In the first volume of his lectures' books, Feynman tries to mathematically describe that in a particular gas ( particular because in this case is the sum of 2 monoatomic gasses with different masses) as a result of the collisions beetween atoms it will be equally likely to find any pair moving in any direction in space , and to mathematically explain this, says: The idea is that any area on a sphere centered at a collision point will have just as many molecules going through it as go through any other equal area on the sphere. So the result of the collisions will be to distribute the directions so that equal areas on a sphere will have equal probabilities. The paragraph in question is the 39-13 of the first volume. It's about the relation beetween kinetic energy and temperature. I have 2 questions: 1) Could you explain me better what this sentence means? 2) If you know, why he use this example to explain the motion of the piston beetween 2 monoatomic gasses ?