Give a brief, nontechnical description of what the digestive system is. You can say, "The digestive system is made of the parts of the body that work together to turn foods and drinks into energy that we need to live and grow."
Explain that people need good foods and plenty of water for our bodies to be able to work and play.
Make clear that foods full of nutrients and fiber are especially important to eat in order to keep our digestive systems working properly. Take children to a grocery store and ask them to point out foods that are good for our digestion.
Help preschoolers understand the basics of digestion by putting your lessons in terms that they're used to. For example: The most nutrient-dense and fiber-rich fruits and vegetables are the most colorful ones. Have children make that connection by naming some of their favorite colorful fruits and vegetables.
Teach children about how other factors, such as emotion and environment, can affect hunger and digestion. For example, when they feel nervous, they may lose their appetite. Likewise, when they feel hungry, they may be grumpy until they eat.
Use a simple diagram of the main organs involved in digestion. Start with the "watering mouth" effect when food is seen or smelled; this is saliva getting ready to start the process of breaking down food. Continue on to describe the function of the esophagus, stomach, intestines, liver and rectum.
Point out the importance of water to the digestive system. Water helps to break down food in the stomach, but it also is a large component in the layer of mucus that acts as a barrier between stomach acid and the stomach wall. Follow this up by encouraging children to drink water throughout the day.
Explain the function of each food once the body digests it, and illustrate these facts with examples. You might say, "Protein builds and maintains body tissue, like muscles. So, after a hard soccer game or swim practice your body will be using protein to help repair and build the muscles that you've just used."
Use the four food groups (fruits/vegetables, dairy, grains, meats) to demonstrate what an ideal meal plan may look like, and the problems with eating too much from one group and not enough from another. The U.S. Department of Agriculture has devised the My Plate system to teach children about how much of each food group they should include in their everyday diet.
Have children construct their own meal plan and explain their choices.