I have had an interesting thought. As you may or may not know, Higgs particles are thought to be responsible for, (the "illusion" of), mass. According to Newton, unless an external force acts on a body it will continue to travel in a straight line at a uniform velocity or stay at rest, (or words to that effect). This leads me to the conclusion that Higgs particles resist changes to a bodies acceleration. My theory is this: Firstly: For each Higgs particle that acts on a particle, the mass of that particle is increased by a fixed quantized amount of mass, (ie: a very small fundamental amount, eg: 10^-100 Kg). Secondly: When particles move faster, they pass through more space and therefore more Higgs particles act on the particles per unit time. Therefore the mass of the particles increases slightly with velocity. Thirdly: All Higgs particles act on particles for a fixed amount of time, regardless of how fast the particle is moving. This gives the effect of accumulating mass exponentially while the velocity increases uniformly, although I admit that I haven't quite twigged where the speed of light limit comes in yet. It does however explain why the mass stays constant with velocity, because for any one particle travelling at a fixed velocity it will always have a fixed number of "hangers on". Feel free to pick holes in it, add to it, make suggestions or point out things that I've over-looked etc. All I ask is that you just don't bury me in maths. If you have an alternative theory on why mass increases with velocity, or even why mass exists at all, I'd love to hear it.