 Bondi, in his book "Relativity and Common Sense", first published in 1964 and based on articles published in "The Illustrated London News" in 1962, reverses the order of presentation.He begins with what he calls "a fundamental ratio" denoted by the letter formula_1 (which turns out to be the radial Doppler factor).From this he explains the twins paradox, and the relativity of simultaneity, time dilation, and length contraction, all in terms of formula_1.