Turn your smartphone sideways and the screen can rotate almost immediately. Tilt it while playing a game and the game may respond to your movement. Your phone can detect these changes because it contains tiny motion sensors called accelerometers. Acceleration describes how quickly velocity changes. An object accelerates when it speeds up, slows down or changes direction. Modern phones contain extremely small devices called MEMS accelerometers. MEMS stands for microelectromechanical system. These devices contain microscopic structures that can move by tiny amounts. When the phone accelerates, the structures inside the accelerometer respond. Electronics measure this movement and convert it into an electrical signal. Accelerometers usually measure along three perpendicular axes, allowing the phone to detect motion in three dimensions. An accelerometer can also help a phone determine its orientation relative to gravity. When the phone is supported rather than freely falling, the sensor responds to forces acting on its internal structure. Software can compare measurements along different axes and estimate which way the phone is tilted. Phones usually combine this information with other sensors. Gyroscopes measure rotational motion, while magnetometers can detect Earth's magnetic field and act as part of an electronic compass. Combining information from several sensors gives a more accurate picture of the phone's movement and orientation. Similar sensors are found in drones, games controllers, cars, smartwatches and fitness trackers. A drone, for example, continuously measures its motion so that its control system can make tiny adjustments and remain stable. Acceleration is therefore not just something calculated in a physics lesson. A sensor only a few millimetres across may be measuring it many times every second inside your pocket.