FLASH! Lightning streaks across the sky! A few seconds later, the thunder arrives. BOOM! In Real Life, thunder is the sound caused by a lightning strike. It has a delay because sound travels slower than light, and so the farther away the lightning strikes, the longer it takes for the thunder to arrive. If you hear thunder at the same time of a lightning strike, it probably means that you're inside the eye of storm. Most writers, when they avert the trope, will generally include the counting to show the distance of the storm.