Lightning is flash of light and thunder is a sound. Since sound travels at a much slower speed than light, the time it takes between seeing the lightning and hearing the thunder can determine how far away the lightning strike was. There's a big misconception with this, however. I learned in school, as many kids learn in school or from their parents, that if you count the number of seconds between the flash and the thunder, then that is equal to the number of miles the storm is from your location. In other words, if you counted 3 seconds between flash and thunder, the strike was 3 miles away. This is not true. In fact, the storm is much closer than 3 miles.
A more accurate rule of thumb is to take the number of seconds between the flash and the thunder and divide by five. If you count three seconds between the flash and the thunder, the storm is 3/5 miles away. That's much closer than 3 miles!
This is a neat little exercise to do with kids. It introduces them to the difference in light versus sound speed and allows them to use their knowledge to figure out the distance to the storm. It also reinforces their math skills. Once you start dividing by 5, a little bit of math is used.