The Paris Gun was a weapon used by Germany during World War I. It was the biggest gun ever built and it could fire shells up to 81 miles (130 kilometers) away! It was simplely a large version of a normal gun, with a longer barrel so the shells could travel very far. The gun itself was very tall, more than 80 feet (25 meters) tall. It was so big that it had to be mounted on a railroad track to be moved around. The Paris Gun was used to attack cities like Paris, France from far away, so people in the city couldn't defend themselves.