Allow a fruit or vegetable, such as a bell pepper, to decay by letting it sit on a counter or shelf for a few days until it is soft and smells rotten. Place it on a piece of newspaper next to a fresh fruit or vegetable of the same variety, such as a fresh bell pepper. Observe the two items with the preschoolers and note the similarities and differences between the fresh produce and the decayed produce. Encourage descriptive observations and thoughtful explorations of the mechanisms behind decay.
Show preschoolers a bird nest or an image of a bird nest. Ask them how they imagine the bird builds the nest and what it uses to build it. Give children bird nest materials, such as twigs, leaves and moss. Allow them to attempt to build a nest, and when they find it difficult to do, ask them why it is difficult for people but manageable for birds. Provide them with paper bowls and glue, and help them to glue the materials to the bowl to look like a bird nest.
Fill one bowl with ice water and another with lukewarm water. Ask the preschoolers to feel the outside of the bowls and to predict whether the water inside is hot or cold or in between. Have them put their fingers in the water and talk about how it feels. Then show them a thermometer -- save discussion of numbers for another time, and instead, show them how the red line in the thermometer becomes longer or shorter depending on what bowl it is in. Ask them what would happen if it was placed in food just out of the oven, or in the refrigerator.
On a chalkboard or whiteboard, draw a very big map that traverses oceans, deserts, forests, fields and ponds. Name some animals and ask the preschoolers where they would live on the map. Give the children construction paper and crayons in order to draw animals. Cut out the animals and ask the children where to tape each one on the map. Ask them questions such as: Why does a lizard live in the desert? Should they approach a large dog we don't know?