I was explaining SR to someone today. I explained why c is the threshold. I think my explanaiton is invalid - is it? *I said that with x amount of energy, a lower mass would go faster. thus something with no mass would have no inertia, thus would go the fastest. *Also something that requires no medium would also go faster than something that does. Well the guy buyed it (for now) But I better come up with a better explanation. So can anyone quickly explain or give me a link to tell me why c is the limit?