Putting on weight

Hi there

I was talking with a friend.  We were discussing putting on weight.

"How come, " pondered my friend, "that I stand on the scales, clothed, clutching a packet of chocolate biscuits to my bosom and I will be a certain weight, right?  Then I scoff down that whole packet of biscuits, stand on the scales again - same clothes, same scales - and I'm suddenly as heavy as an elephant?"

I pointed out the obvious.  "Because you've just eaten a whole packet of biscuits?"

"But the biscuits should weigh exactly the same inside me as they do outside of me.  If the biscuit packet weighed 200 grams, I should have put on 200 grams. Not a whole 1 kg!"

Ah, the great mysteries of the universe ....  but more to the point, did my friend actually conduct such an experiment?  We senior citizens really should get out more?