This little science experiment will introduce students to the idea of chemical interaction and density. In a small bowl or container, pour a quarter cup of baking soda. Follow this with a full cup of vinegar over top of it. (If you want to use a larger or smaller volume, just keep the ratio one part baking soda to four parts vinegar.) The toddlers will be amazed as the baking soda and the vinegar begins to bubble and fizz during the interaction. You can encourage the toddlers to blow on the fizz. Watch as more bubbles come to the top, while the carbon dioxide (expelled breath) is forced under those bubbles to the bottom. As a craft project, make a clay volcano first, and then perform this interaction inside. Other ideas adding soap and food coloring to the vinegar before adding the baking soda. This will create more "bubbles" and add a different color to the reaction.
Many children are fascinated by food. Though they might not yet understand where foods come from, you can dovetail a lesson about farming or dairy nutrition into this science experiment. Pour heavy cream into a jar. Secure the lid tightly, and begin to shake the jar. (As the adult, you will need to assist with this.) After about 10 minutes, you and the toddler will see clumps of liquid start to form. This is actually butter. The liquid part left over is butter milk. You can even spread the butter on to a snack for the toddler. Just be sure to refrigerate the butter when you are done.
Young toddlers might be taken with the many colors of flowers around the house or yard. You can teach them how flowers feed with a simple demonstration that they can keep as a memento. In a small plastic cup, fill it half way with water, and then add several drops of food colors. Instruct the toddler to place the head of a light-colored flower into the food coloring. Check back in an hour or two. The food coloring will start to travel along the veins of the flower and give it color. You can explain that flowers need water to grow and live and that the coloring of the flower represents where the water is traveling.
For a beginning lesson in biology, go out into the yard with your toddler and use one of their plastic shovels to dig a few scoops of dirt into a plastic cup. Then, on a picnic table or deck, spread out the soil on a piece of wax paper. Provide the toddler with a magnifying glass to see all of the various creatures found in the soil. As they spot bugs and worms, ask them to describe the various creatures. Some might have wings, many legs, or appear to have no legs at all. You can educate the toddler on the fact that there are many creatures both big and small that share the planet.
Plant a butterfly garden as a spring or summer project to teach your preschooler about insect life, giving them an opportunity to realize that the caterpillars they might see crawling through the dirt or on plants will turn into bright, beautiful butterflies. You can make the garden as big or small as you like by simply buying a few pots, potting soil, and host plants that are known to attract butterflies. Host plants include "Husker Red" penstemon, tropical milkweed, Rue, sneezeweed, lupine "'Russel' hybrids," turtlehead, butterfly weed, "Silver Brocade" artemisia, violets and pansies. After purchasing these plants, you and your preschooler can spend an hour planting them according to the plant directions. Give her the job of making sure the flowers are watered properly as she looks out for butterflies.