You can demonstrate the concept of growth by having your students grow living things in the classroom. They can grow plants from seeds under different conditions of light, temperature and moisture. Once the plants begin growing, the students can compare the rates of growth under the different conditions. You can vary this activity by doing the same thing with moistened bread to compare mold growth, or do both plants and mold simultaneously.
Your students can learn about natural selection through a simple activity involving dried beans and construction paper. Have the students place 50 black beans, 50 lima beans and 50 kidney beans on a single sheet of green construction paper. Ask them to close their eyes for 30 seconds, then open them and pick up the first bean that they notice. Tell them to set the bean aside, close their eyes for 10 seconds, then pick up another bean. The students should do this 10 times and write down the results, then do the same thing on a sheet of black construction paper. Ask the students which beans fared better and why, and what would happen if the experiment continued.
Teach your students about the carbon cycle and ecology at the same time by having them make their own compost. The students should combine grass cuttings, vegetable scraps and fruit scraps with a small amount of garden soil and enough water to moisten the mixture. Have them put the mixture into a closed container with air holes. By the end of two or three months, the mixture will change into dirt.
A classroom model of the water cycle is simple to make. Shape some modeling clay into the form of a mountain with one steep side and one sloping side. Set the clay mountain in a small aquarium with the steep side facing the closest end of the aquarium. Add water until a quarter of the clay mountain is underwater. Cover the aquarium with plastic wrap, place several ice cubes on the wrap above the clay mountain and turn a lamp on directly above the aquarium. After 15 or 20 minutes, have the students describe any changes they have observed.
Your students can gain insight into the concept of density by setting a can of citrus-flavored soda and a can of diet citrus-flavored soda in a container of fresh water. Unless the water has a lot of minerals dissolved in it, the diet soda will float and the regular soda will sink. Discuss the reasons for this with the students, then mix some salt into the water and see what happens.