Why does the Earth spin?

There are various phenomena that are caused by movement of heavenly bodies. We observe different seasons throughout the year because the earth is at different positions relative to the sun as it revolves around it. Tides are caused by the relative position of the earth and the moon. Similarly, day and night are caused by the earth's rotation. But why is the earth spinning?

Earth, and other planets, rotate about their axis - or spin - due to inertia. Inertia is the tendency of objects at rest to stay at rest, or to move at a constant rate when they're moving unless external force is applied. Hence, the earth is spinning because it was spinning when it was formed, and has just continued to do so due to inertia.

To understand why it started spinning in the first place, one has to go back to the birth of the entire solar system. Dust and gas are what started everything. The cloud began to collapse and the gravity in the center started pulling things in as it collapsed, compacting dust and gas forming stars-- and planets. As this happened, the formed bodies started spinning faster. There was angular momentum, or the tendency of objects to spin. Hence, when earth was formed, it was spinning, and it hasn't stopped since.

Interestingly, it is believed that the earth hasn't always spun at the same rate. It was slower. However, it was hit by a large object - which ultimately caused the formation of the moon - and increasing the earth's speed in the process.