You can measure the inside of something by using a variety of methods, such as sonar, radar, or ultrasound Each method uses a type of energy (sound waves in the case of sonar and ultrasound) that travels through matter and then is reflected back to the source. By measuring the time it takes for the energy to travel to the target and back, and then by converting that time into a distance, you can calculate the size (and sometimes shape) of an object.