Hi everyone, many (sigh), many years ago, when I was in high school, I recall a little discussion we had with some classmates, which I gave little thought to at the time, but thinking back to it now, could never really figure out. It was about relativity. The story was that nothing could reach the speed of light, because the energy needed to achieve that would be infinite, and the mass of the object would then also be infinite. And one of the guys said, what about things that *do* travel at the speed of light, like photons? Haven't they got any mass? The obvious answer (not coming from me) was that, as they were waves rather than particles, indeed they had no mass, they were only energy. And he said, but if mass and energy can turn into one another, and particles and waves are the same (we had probably just been taught the De Broglie thing), then anything that has energy has also got mass, so how can photons travel at the speed of light and still have finite mass? I suppose such questions don't even arise in physicists' circles, but for an outsider like myself they can make matters quite puzzling. Can you please explain where my classmate's reasoning was wrong? Thanks!