So I'm studying for the Physics GRE and I came upon a question in the sample test that reads: A lump of clay whose rest mass is 4 kilograms is traveling at three-fifths the speed of light when it collides head-on with an identical lump going the opposite direction at the same speed. If the two lumps stick together and no energy is radiated away, what is the mass of the composite lump? (A) 4 kg (B) 6.4 kg (C) 8 kg (D) 10 kg (E) 13.3 kg Naturally I'd think the answer was (C), but the answer key gives (D) as the correct answer. Why? Obviously it has something to do with relativity. What's interesting is that the speed of the second lump relative to the first lump is NOT 6/5c because it's impossible to exceed c. Right? But what I don't understand is how it affects mass. It looks like we would some how be converting energy to mass in this collision, since we can't create mass. So how do you calculate how much energy is converted to mass, and how much mass we get out of it?