Jack and John are twins, (They're each the other one's twin, we'll say they're identical.) Jack was born in May. John was born in February. [hmm, usually the next part is: When Jack was 20 John was 20 too, (sounds like 22, hence the riddle) but it doesn't work when i spell it... awell...] When Jack was 30 he legally married his mother. How is all that possible???