A report today said that Science and Maths exams are harder then arty-farty ones. As a Literature graduate therefore (!) not only am I in the less intelligent half of the population but am also technically, numerically and common-sense wise illiterate. So though I can recite the whole of Othello by heart, know how Keats died and know how to choose a good book to read to the kids I don't understand tyre sizes. I have tried to help myself - I've visted Sheldon Brown's site, the CTC site and posts here. But I have only 2 questions which I need an answer to. Please. I have two bikes - one is a wheel size 700x23 (langster), the other a 700x28 (sirrus). Does the 700 refer to the diameter? What does the other number refer to? Secondly, can I interchange the inner tubes on these?