What causes thunder to follow lightning during a storm?

Thunder follows lightning because of the difference in the speeds of light and sound. Lightning is essentially a massive electrical discharge, and the flash travels at the speed of light, reaching your eyes almost instantaneously. The intense heat from the lightning bolt rapidly superheats the surrounding air to around 30,000 Kelvin, causing it to expand explosively. This rapid expansion creates a shockwave that we hear as thunder, which travels at the much slower speed of sound, roughly 343 meters per second. The farther away the lightning strike, the longer the delay before you hear the thunder. A common rule of thumb is that every five seconds between the flash and the thunder equals about one mile of distance.