Pop Rocks Candy is like other hard candy made from sugar, lactose (milk sugar) corn syrup, and flavouring. The trick to the “popping” lies in its carbon dioxide processing. The ingredients are heated together and boiled. The hot sugar mixture is mixed with carbon dioxide gas at about 600 pounds per square inch (PSI). The carbon dioxide gas forms tiny, 600 PSI bubbles in the candy.
Once the candy cools the pressure is released and the candy shatters, but the pieces still contain the high-pressure bubbles (you can see these bubbles with a magnifying glass). When the candy is placed in the mouth, it melts and releases the bubbles with a loud POP! What the eater hears and feels is the 600 PSI carbon dioxide gas being released from each bubble. (PopRocksCandy.com)