Use a wilted celery stalk with a small slice cut off the bottom. Fill a glass half-way full of colored water and put the celery in. When they see that the wilted stalk stands up again, you can talk about how plant and animal cells are different. Animal cells are round and the nucleus, which contains the DNA, are in the center. Plant cells are more square in shape and have a cell wall. The nucleus is not in the center, but have something called the central vacuole in the center. This vacuole is like a water balloon and when the cells have plenty of water, this vacuole fills up and presses against the cell wall, causing rigidity in the cell. This rigidity in the cells makes the plant in general stand up straight. It is the water leaving the cells that make plants wilt.
source: Exploring Creation with General Science, Jay Wile