   Collision response
  I'm trying to make my player bounce off an object from the direction he hit it. So I am tryingVector2 normal = position - lastPostition;normal.Normalize();position -=  normal *2;But position.X and position.Y end up being NaN. But if I don't Normalize, I don't get NaN. The problem is, if I don't normalize, it's harder to fine tune the bounce. So why would I be getting NaN?   
  normalise uses a float division. You're probably dividing by zero.   