What could be practical about lightning? Many folks knew what wasn't practical: having your house burn down because it was struck by lightning. Franklin thought he could help. He knew that lightning usually hit the highest part of a building. He also knew that the electrical current in lightning could start a fire. So he invented the lightning rod. A lightning rod is made of metal and is attached to the highest point on a house. The lightning hits the rod instead of the house, and the electrical current from the lightning goes into the ground and leaves the house undamaged. Franklin thought the lightning rod was his most important invention.