I like to start this lesson with an electric skillet, an extension cord, and an old sheet spread on the floor. Put the electric skillet atop the sheet and turn it on high. Children can sit around the sheet (not on it) and observe the action. The sheet keeps the floor clean and provides a safety boundary.
Observe what happens to water when it gets hot. Once the skillet is hot, use an eye dropper or straw to add a drop of water to its surface. It’s more dramatic to add the drop from a height above the skillet. Students usually delight in the sound of the hissing water and the sight of the water sizzling and disappearing. What happened to the water? Where did it go? Students often want to see this repeated several times. You might want to add a larger amount of water on subsequent demonstrations so they can watch it for a longer time.
Explain to students that heat causes the water to expand to the point where it bursts and turns into steam. I like to draw a slo-mo cartoon of this on the board: a spherical drop of water hitting the hot surface, growing larger…and larger…and bursting like a firecracker. Energy is released when the drop turns into steam.
The energy released with a single drop of water isn’t much, but the energy released with large amounts of water can be quite large. This energy has been used to power large machines. Show children pictures of steam trains, large ocean-going ships, boilers on tug ships. Find the heat source and the boilers on these pictures.
Popcorn. Give each child a kernel of unpopped popcorn. Ask them to describe its shape, size, texture, smell, even the sound it makes when dropped onto a hard surface. Some children may have only popped corn in a pouch and never seen what unpopped kernels look like. Have they ever popped corn at home? How did they do it? (they heated it in various ways). What causes popcorn to pop? Why doesn’t corn on the cob pop? There’s something different about popping corn, but it’s clear that heat is involved.
Demonstration: add a small amount of corn oil to the hot electric skillet. The oil helps spread the heat around the kernels. Then add one or two popcorn kernels. Keep the number of kernels small to allow the students to concentrate. Observe the kernels closely. They may begin to jiggle; then pop. Ask the students again why they think the popcorn popped. After waiting for answers, you might suggest there is something inside the kernels that explodes when it gets hot. Return to the previous demonstration about steam. Inside each kernel is a small amount of water. When it gets hot, it turns to steam with an explosion, causing the kernel to pop inside out. Another factor is the tough skin on the outside of the kernel, which holds the water in place until it is heated sufficiently to explode the kernel.
Enjoy: Add more oil if necessary, then add enough popcorn to serve the entire class. Leave the lid off the electric skillet for maximum fun. Stir the popcorn as it heats, to keep it from burning. Instruct the students to keep off the sheet, and to not pick up any popped corn. It will be hot! The teacher or parent can collect all the popped corn from the sheet when the skillet is empty. As the corn pops, think of the power released. Watch the height and distance the corn travels when it leaves the skillet. After the corn has popped, compare it to the unpopped corn the students had at the beginning. How has it changed?
Connections to other cultures and history:
Corn cobs with remnants of popped kernels have been discovered in the mountains of Peru, dating from approximately 6000 years ago. That corn was probably discovered accidentally when roasting cobs directly over the fire. Ancient Peruvians may have begun selecting and saving kernels of this type of corn for later planting. Ancestors of today’s Pueblo Indians knew how to pop corn. Popped corn dated from 1000 years ago was found in a cave in Utah where they lived. By some accounts, it was popcorn that native Americans offered to European pilgrims upon their arrival in the Americas.
Street vendors in Afghanistan and India use an unusual method of heating their popcorn: roasting it in skillets of sand set over a fire. I encountered this method while traveling in Afghanistan and at first could not believe what I saw. But my friends insisted it was true, allowing me to touch the cooled sand in the skillet – to the amusement of the street vendor. The popped kernels rise to the top of the sand and are cleansed of it by shaking the kernels through a strainer. In fact, further research on my part has led to my discovery that Iroquois Indians heated their popcorn in this way. French Canadian explorers were served the popcorn fresh, and in popcorn soups.