As far as I understood the relativity: 1. Massless particles travel at the speed of light. 2. As the speed increases, so does the mass of the moving body. It goes to infinite as the speed gets close to that of light. My questions: Why would particles with infinite mass (theoretically) move at the same speed as the massless particles? What's the connection between these two kinds? You may say, it is impossible to reach infinite mass, but anyway, why does mass of let's say electron moving at 90% speed of light gets very big, while the mass of the photon which travels at 100% of the speed of light is zero?