The Physics Behind The Firecracker Scene: Home Alone 1

In case you've forgotten the scene...

The physics behind firecrackers...
What Kevin most likely does not know is that in lighting the firecrackers, he was lighting a fuse to open a circuit. When this short length of wire is lit, it will melt and separate since it is under excessive current. This fuse is connected in a series circuit with the other components in the firecracker to be protected from over current, so that when the fuse blows/opens, the circuit will be opened and stop current through the firecracker, thus exploding and making a great scare tactic against burglars like Marv. 

Physics can also explain the crackling and various pitched sounds the firecrackers made. The lower the pitch that the firecrackers made, the lower the frequency the sound waves were travelling at. On the other hand, the higher the pitch that the firecracker made, the higher the sound waves were travelling at.