Kids can learn about polymers when they make this gooey concoction. Mix equal parts cornstarch and water in a large bowl. The mixture will become sticky and dough-like. To thin the substance add more water, or to thicken it, add more cornstarch. Once your slime is smooth you can add food coloring of your choice to make it more realistic. As children play with the slimy substance, discuss how it has properties of both a solid and a liquid.
You can make decorations for a Halloween party while observing how fruit dries out. Start with an apple and remove the skin on the outside, leaving the stem attached. Coat the peeled apple with lemon juice to minimize oxidization and browning. Then begin carving the apple into the shape of a face. To draw the water out of the fruit and begin shrinking it, soak the apple in salt water for about 24 hours. Hang it up to dry for about a week and then decorate it to look like a real head.
For younger children, a pumpkin activity is a great way to introduce the scientific process. Gather an assortment of pumpkins of different shapes and sizes. Ask the children to predict if each pumpkin will sink or float when dropped into a tub of water and record their guesses. Invite them to drop their pumpkins into the water and observe it they sink or float and check it against their predictions. Have them brainstorm why the pumpkins float instead of sink, and lead them to conclude that it is because they are a hollow fruit.
Write secret messages and astound children with a special kind of color-changing paper, called Goldenrod Paper. This normally yellow paper turns red when exposed to basic solutions like ammonia. Use a piece of wax to write a message on the Goldenrod Paper and then wipe a cotton ball dipped in an ammonia-water solution across the paper to expose the hidden words. Or make a "bloody" handprint on the paper by spraying your hand with the ammonia-water solution and then pressing your hand against the Goldenrod Paper. Kids will learn about acids and bases in this activity, as they observe the paper turning back to yellow once it exposed to an acidic substance like vinegar.