In earliest times, men considered lightning to be one of the great mysteries of nature. Some ancient people believed that lightning and thunder were the weapons of the gods.  In reality, lightning is a flow of electricity formed high above the earth. A single flash of lightning 1.6 kilometers long has enough electricity to light one million light bulbs . The American scientist and statesman, Benjamin Franklin, was the first to show the connection between electricity and lightning in 1752. In the same year he also built the first lightning rod  . This device protects buildings from being damaged by lightning. Modern science has discovered that one stroke of lightning has a voltage   of more than 15 million volts  . A flash of lightning between a cloud and the earth may be as long as 13 kilometers, and travel at a speed of 30 million meters per second. Scientists judge that there are about 2,000 million flashes of lightning per year. Lightning hits the Empire State Building in New York City 30 to 48 times a year. In the United States alone it kills an average of one person every day. The safest place to be in case of an electrical storm is in a closed car. Outside, one should go to low ground and not get under tress. Also, one should stay out of water and away from metal fences. Inside a house, people should avoid open doorways and windows and not touch wires or metal things. People once thought lightning came from   _  .
the gods the earth the sky nature