Although spoiled food is something adults usually try to avoid, it can be an interesting phenomenon for children to observe! To do this, get four pieces of bread. Put two of them in a bag; leave the other two on a dish, uncovered. Every day or two examine the bread using both your eyes and your nose. Talk about what is happening to the bread.
Try the same thing with pieces of a few different fruits and vegetables, like apples, peaches, and peppers. Every day or two examine them to see the changes in their shape, size, and color.
For an extension of this activity, try making your own mini-compost pile. Place a layer of dirt several inches thick at the bottom of a cardboard box. After eating the edible parts of several vegetables and fruits, help your child cut the remains into small pieces. Place them in the dirt and mix well. For some real fun, dig up some worms and add them to your mini-compost pile! Keep the box in a warm spot and spray it with water occasionally. (If you don't add meat, dairy products, or sugar to the mix, you shouldn't have a problem with unwanted smells or animals.) Over a period of weeks, watch what happens to the small pieces of food. Once you no longer see the food, you can use this nutrient-rich dirt as fertilizer for your plants.