A lot of things spin – bike wheels, footballs, hard disks in your computer, and even the Earth – they’re all twirling around. Spinning things have inertia, which means they keep spinning unless something slows them down. Bike tires keep spinning until you put on the brakes. A football spirals through the air until you catch it. The Earth keeps on spinning 24 hours a day, seven days a week. It’s been spinning for over four billion years.