Break up three to four Mentos candies into small pieces. You will need pieces small enough to fit through the soda can opening. Cutting or chopping the pieces into four sections or quarters will be sufficient.
Place the pieces in a small test tube. The test tube should be small enough to fit into the soda can opening. Make sure the test tube has no other materials in it and is completely dry.
Place an index card at the top of the test tube. Flip the test tube over so that the open end is at the bottom and the candies are prevented from falling out by the card. Momentarily stand the tube to the side.
Open the can of soda and remove the tab. Pour one-fifth of the liquid out. Cover up the opening of the can with your fingers and shake the can gently enough to create a fizz in the liquid but not enough to make it rise out of the can.
Place the index card and test tube over the opening. Hold onto the test tube and quickly remove the card, allowing the Mentos to fall into the can.
Step back from the can but hold onto the bottom. Make sure that your face is not above the can. Watch the volcano of soda shoot out of the can. Repeat with more Mentos if you would like to create another explosion.