What causes thunder after you see lightning strike?

When lightning strikes, it superheats the surrounding air to around 30,000 Kelvin almost instantly, which is about five times hotter than the surface of the sun. This extreme heat causes the air to rapidly expand and then contract, creating a shockwave that we hear as thunder. You see the lightning first because light travels at 186,000 miles per second, reaching your eyes almost instantaneously. Sound travels much slower, about 1,100 feet per second, so the thunder takes time to reach you. This is why you can estimate how far away a storm is by counting the seconds between the lightning flash and the thunder, then dividing by five to get the approximate distance in miles.