Having read a whole lot of discussions about the "twin paradox" in this forum, I have a question that probably only serves to shine a bright light upon my ignorance... Clocks running slower at near-lightspeed, OK. Sounds reasonable. I'll buy that. But why does this mean that the space-travelling twin is also aging slower than is his Earth-bound brother? In what magical way does relativistic time dilation "slow down" biological metabolism???