This article from brighthub.com is a great way to get young children to start to understand biology. It involves cooking to learn science. Students design a set of criteria to determine if something is alive.

"A great way to explain biology to elementary and early, middle-school age children is to get them cooking. In particular, baking bread is a fabulous way to introduce children to biology. Yeast, the ingredient in bread that makes it rise, is a bacterium. Proofing the yeast brings it to life and sets off a series of biological functions. If the yeast reactions are lacking or missing, you end up with flat bread or matzo, not a loaf of bread."