kynan: "mummy, how heavy is air?"

me: "err, go ask daddy." :)

kynan: "daddy, how heavy is air?.... does it weigh nothing?"

dh:  "err, no..... from here to outer space is the same as 10metres of water pressing down on u."

kynan: "aaah.... so that's why we can't fly up....the air pressing down on us is too heavy!"

***bravo to dh's scientific answer!... tho i admit i am still none the wiser :) ***