It's to do with concentration gradients.
Water moves across the membrane from an area of low concentration (of sugar or whatever) to an area of high concentration, in an attempt to equalise.
Quite why it does this I can't remember...
(
supermoore: HUNG,
Fri 18 Jun 2004, 23:42,
archived)
Pfft, it's not like it THINKS about it or anything,
it just does. Entropy in action.
(
goat thinks you're shit,
Fri 18 Jun 2004, 23:46,
archived)