What cause the pop in the cracker when you pull it?

The pop in a cracker occurs due to the sudden release of built-up tension in the material. When you pull the ends of the cracker, the tension causes a quick rupture of the inner layer, which contains a small amount of explosive material, typically a friction-sensitive compound. This rapid expansion of gases and the breaking of the cracker's structure create the characteristic popping sound. The design of the cracker ensures that this reaction happens quickly and loudly.