Photo: CC0 Public Domain
Some Christmas crackers use a chemical called silver fulminate which is a very shock-sensitive substance. The cracker contains two long, narrow strips of cardboard, one of which is painted with some silver fulminate and the other contains a sandpaper-like material. When two people pull on each side of the cracker, these two pieces of cardboard come in contact with each other and the friction from the abrasive side detonates the silver fulminate, giving that popping sound.