Instead of working with water, which might be your first inclination, I found that when working with my son, using fun objects that they'd want helps them to better understand if a container was full or empty. For example, take a cup and fill it with marbles. Take the marbles out and put them away. Or those little monkeys in a barrel are another fun thing you can "play" with where putting things in and taking them out makes sense to the kids. I think with this, my daughter, who is much younger, even understands and says "Oh, no! All gone!"