  The k-calculus was advocated by Hermann Bondi as a means of explaining
special relativity using only GCSE level mathematics and ideas. We review the
central derivations, using proofs which are only a little more elegant than
those in Bondi's books, and extend his development to include the scalar
product and the mass shell condition. As used by Bondi, k is the Doppler red
shift, and we extend the k calculus to include the gravitational red shift and
give a derivation of Newton's law of gravity using only 'A' level calculus and
basic quantum mechanics.
