Growing a plant can show children the basics of the life cycle. Insert a few toothpicks into a sweet potato and place it into a jar with the toothpicks on the bottom. Fill the jar with water so that the bottom of the sweet potato is immersed. Place the jar in a sunny place. Water it daily, keeping the water at the same level. You should see sprouts within a few days. After a few weeks, you should have several shoots from the sweet potato. Move it to a pot and bury the sweet potato in dirt. Continue watering the plant and within a few months, you will have a full-grown sweet potato plant.
This will teach preschoolers about the basic parts of the plant and plant reproduction. You will need a large printout of a flower. On another sheet of paper write down the various parts of a plant, including roots, stem, leaves and flower. Cut the parts of the plant into strips and put double-sided tape on the back. Put double-sided tape on the printout at the spots that match the plant parts. Have the children put the different parts onto the printout and discuss what the parts of the plant do.
Children often eat plants and may not even realize it. Show the children various fruits and vegetables. Ask them about their favorite ones. Talk to them about how the various foods come from plants and how some of them are actual plants. Use root vegetables such as potatoes, leaf vegetables such as spinach or lettuce, tree fruits such as oranges or apples, and bush and ground fruit such as blueberries and strawberries. Show the children the seeds in the fruit and show them how spinach is the leaf of the plant.