Gather the items you need before you start near where you will observe the demonstration of cloud formation. The place should be a room or closet that is large enough for all of the people who are going to be watching and that can also be made quite dark. Make sure that your glass jar is see-through. If there is a label on the side of the jar, remove it so that people can easily watch the cloud that will be forming inside the jar.
Heat a cup of water until it boils. If you heat the water on the stove transfer it from the saucepan to the pitcher. If you heat the water in a microwave, you can use the setting for cooking a cup of coffee and heat the water in the same pitcher that you will use to make your cloud.
Fill the 9 x 13 inch metal pan with ice cubes. The metal pan will get very cold after just a few short minutes. .
Pour the hot water into the glass jar. Then place the metal pan with the ice on top of the jar so that it completely covers the jar's mouth. Your cloud formation demonstration will work when the heat from the water in the bottom of the jar rises to meet the air that is being cooled from the ice above.
Darken the room. Shine the flashlight into the jar. Watch the cloud form inside the jar. This is exactly what happens in the earth's atmosphere. When cold air meets warm air a cloud will form. This demonstration is a great science fair exhibit that you can repeat on a schedule you post at your exhibit table.