Why do we have sunsets?

Answer 1

Because we have only one sun in our solar system, because Earth is roughly spherical (like a ball), and because light almost always travels in straight lines (except for a little bit of refraction caused by our atmosphere), the sun is always shining on half of Earth, where it is daytime, while the other half of Earth is in Earth's shadow (nighttime). Since Earth is always rotating on its axis, part of the line between the daytime half and the nighttime half is rotating into Earth's shadow and part of the line is rotating out of Earth's shadow into the sunlight. At the places that are rotating into the sunlight, the sun appears to be rising, and at the places that are rotating into darkness, the sun appears to be setting.