The apparent movement of the Sun across the sky during the day is caused by the Earth's rotation on its axis.

Here's a breakdown:

* Earth's Rotation: Our planet spins on its axis, completing one full rotation approximately every 24 hours. This rotation is what defines a day.

* Perspective: As the Earth rotates, we, as observers on the surface, perceive the Sun as moving across the sky. In reality, it's the Earth that's moving. Imagine being on a carousel. As the carousel spins, you see the scenery around you move, even though you are the one moving.

* Direction of Rotation: The Earth rotates from west to east, which is why the Sun appears to rise in the east and set in the west.

Important Note: The Sun doesn't actually move across the sky in a straight line. Its path is curved due to the Earth's tilt on its axis and the Earth's orbit around the Sun. This is why we experience different lengths of days and nights throughout the year.