What causes thunder to follow lightning in storms?

Thunder follows lightning because of the difference in the speeds of light and sound. Lightning is essentially a massive electrical discharge, and it superheats the surrounding air to around 30,000 Kelvin almost instantaneously. This rapid heating causes the air to expand explosively, creating a shockwave that we hear as thunder. Light travels at about 300,000 kilometers per second, so we see the lightning flash almost instantly. Sound, however, travels only about 343 meters per second through air, so it takes roughly 3 seconds to cover each kilometer between you and the storm. The further away the lightning strike, the longer the delay before you hear the thunder.