Here's why:

* Earth's Rotation: Earth spins on its axis, completing a full rotation roughly every 24 hours. This rotation is what causes day and night.

* The Sun's Position: The Sun is the source of light and heat for Earth. As Earth spins, different parts of the planet face the Sun.

* Daylight and Darkness: The side of Earth facing the Sun experiences daylight, while the side facing away from the Sun experiences darkness.

Think of it like a giant globe spinning with a flashlight shining on it. The part of the globe that the light hits is illuminated, while the opposite side remains dark.