Using clay or play dough, build a model volcano inside a box or on a board. If you wish, use different colors of clay or play dough to make the volcano look more realistic. For instance, use brown for rocks and dirt, green for grass and trees and red for lava. Make sure to leave enough space in the center of the volcano to fit a can of cola. A box with low sides will help contain the cola/salt mixture as it bubbles out, making cleanup easier.
Open the can of diet cola and place it in the center of the volcano. Though carbonated drinks with sugar in them will also work, the sugar will slow down the reaction as well as leave a stickier mess, making cleanup harder.
Hold the funnel upright, placing a finger over the hole at the bottom. Measure two tablespoons of salt into the top of the funnel.
Still keeping your finger over the bottom of the funnel, place it over the can of diet cola. When you are ready, remove your finger and let the salt drop into the can.
The cola will begin to foam and bubble out and over the sides of your volcano and will continue until nearly all the cola inside the can is gone. Wait until the reaction is finished and then start cleaning up.