This is a simple enough question, but no one has been able to give me a satisfying explanation. Most people compare the momentum's of the bullet and the rock, and say that rock has a higher mass, so it's momentum is greater and it shatters the glass. I don't find this a convincing explanation, and the obvious flaw is that the bullet has a much higher velocity, which would make it's momentum atleast of the same magnitude as that of the rock's. It appears to me that the rock completely transfers it's momentum to the glass, but the bullet only loses a fraction of it's momentum. I can't think of a reason for why this happens.