A sonic boom is created when an object travels faster than the speed of sound. When an airplane reaches the speed of sound, it makes a bang sound or an explosive noise. This is often called "breaking the sound barrier." The visible part of a sonic boom is actually air that becomes squashed by sound waves, which is known as a vapor cone. The thunder you hear when lightning strikes near enough, is also a sonic boom, caused by the lightning moving faster than the speed of sound, and making a sonic boom.