You can demonstrate a volcanic eruption using some simple household items. Kids usually have a great time doing this experiment, and may want to repeat it several times!
To make a big eruption, use a small plastic bottle (the size 20-oz soft drinks come in works well). Fill the bottle halfway (1 to 1.5 cups) of vinegar. To start the eruption, drop a baking soda 'bomb' into the bottle--wrap one tablespoon of baking soda into a small piece of tissue paper, tying the ends with string.
You should see an instant eruption--'lava' will spurt out of the mouth of the bottle. The baking soda, a base, neutralizes the acid in vinegar. This releases carbon dioxide gas, which causes the fizzing action in your volcano.
If you want your volcano to look more realistic, use clay or playdough to make a 'mountain' around the bottle. Or, if you're working outside, you might want to use dirt and pebbles. You can also add red food coloring to the vinegar solution to make it look more like lava.
To learn more about volcanoes, take a look at our Introduction to Volcanoes Teaching Tip.