A gyroscope is a spinning wheel or disc mounted so it can turn freely in certain directions. Once you set it spinning, it strongly resists being tilted or turned, tending to stay pointed in the same direction even when you move the frame around it. This behavior can look like it defies gravity: a spinning gyroscope can balance on a string or on your fingertip, appearing to float in midair.

The key is the spin. When the wheel isn't spinning, it flops over like any other wheel. But start it spinning fast, and it becomes surprisingly stable, fighting against forces that try to change its orientation. You can see this yourself with a bicycle wheel: hold it by the axle while someone spins it hard, then try to tilt it. The wheel pushes back against your hands in unexpected directions.

Gyroscopes help airplanes, ships, and spacecraft know which way they're pointing, even when visibility is poor or there are no landmarks. Your smartphone contains tiny gyroscopes (smaller than grains of rice) that sense when you rotate your phone from portrait to landscape mode. The Hubble Space Telescope uses gyroscopes to point precisely at distant stars, and gymnasts experience gyroscopic effects when they spin through the air, with their bodies resisting changes in rotation.