In September 1783, the Montgolfier brothers, who were French, noticed that hot air rises. They had the idea of trapping it in a big bag, which would then float up. They filled their first balloon by holding it over a fire. Their second balloon even had passengers: a duck, a rooster, and one very scared sheep. Two months later they built a balloon large enough to carry people and put a fire in the base to keep the air in the balloon hot.