A speedometer is an instrument in a vehicle that shows how fast you're going. Look at the dashboard of almost any car and you'll see a dial or digital display showing speed in miles per hour (or kilometers per hour in many countries). When the needle points to 35, you're traveling 35 miles per hour. When it climbs to 65 on the highway, you're moving 65 miles per hour.

Before speedometers were invented in the early 1900s, drivers had no reliable way to know their exact speed. They could only guess based on how fast the scenery rushed past or how loud the engine sounded.

Modern speedometers work through a clever mechanical or electronic connection to the vehicle's wheels or transmission. As the wheels spin faster, the speedometer reading climbs higher. Police officers use radar guns, which are essentially external speed-checking devices, to check how fast drivers are going. Some bicycles have speedometers too, though they're not required by law like they are in cars in many places.