His idea was about electricity and lightning. Franklin noticed several similarities between the two: They both created light, made loud crashes when they exploded, were attracted to metal, had a particular smell, and more. Based on these observations, Franklin thought electricity and lightning were the same thing. A few people shared his belief, but no one had ever tested it.
Franklin wrote up his thoughts on electricity in several letters to a fellow scientist who lived in London. This scientist and other scientists in London thought Franklin's letters contained valuable information, so in 1751 they published them in a little book, Experiments and Observations on Electricity.
One of the letters contained Franklin's plan for how to prove that electricity and lightning were the same. His plan required something tall, like a hill or a tall building, but Philadelphia had neither at the time. While Franklin was waiting for a tall building to be built, he came up with another plan. This one involved a key and a kite.