Lightning is electricity inside a cloud. Scientists do not know exactly what makes this electricity. But they know that the electricity inside a cloud can be as much as 100 million volts. From this extremely strong electricity, a lightning bolt, like a streak of bright light, comes down from the sky. Its temperature can reach 50,000 degrees Fahrenheit within a few millionths of a second. That's almost five times the temperature on the sun's surface. The lightning bolt is very quick. It can move at a speed of 87,000 miles per second. A rocket traveling at this speed would reach the moon in 2.5 seconds. With the lightning bolt, we usually hear thunder, which is the sound of hot air exploding. Lightning and thunder happen at exactly the same time, but we see lightning first because light travels a million times faster than sound.