Why isn't f differentiable at x = 0 in f(x) = |x|?

Because 0 isnt 1 or x for that matter. The only thing you will get by doing math with numbers and letters at the same time is a headache...which is what I have now. Oh and what the devil does Captain Kirk have to do with this...he's in space. I guess you could ask William Shatner though.