If you want to learn some fancy words to teach your kids... keep reading. This experiment can also be done with carnations. Most instructions say leave overnight, but we left ours on the counter a week for a more brilliant/obvious results. Cut a stalk of celery with the leaves, place in a glass of water with food colouring added...check back.
Transpiration, coupled with osmotic pressure from the roots and capillary action in the stem
Another great explanation of capillary action is on the US Geological Survey's Water Science for schools web site.