Calcium chloride is added to water in a plastic bag. As the chemical splits apart, it releases a noticeable amount of heat. An acid-base indicator, cresol red, is then added to the baggie along with sodium bicarbonate (regular, old baking soda) and zipped closed. As the two chemicals react with one another a large amount of carbon dioxide gas is released, the indicator changes color from red to yellow and if you are lucky, the bag will explode. It is the perfect post-mexican dinner reaction; you get very hot, generate a lot of gas and something has to give. Less