* The Earth is a sphere: The sun's light only directly illuminates half of the Earth at any given time. The other half is facing away from the sun, and therefore in darkness.
* The Sun's light travels in straight lines: Sunlight doesn't bend around the Earth. So, the only areas directly facing the sun receive its light.
* The Earth is relatively small compared to the vastness of space: The sunlight reaching us is just a tiny fraction of what the sun emits. Most of the sun's light travels out into the vast emptiness of space, not reaching any planets or stars.
* The atmosphere scatters sunlight: Some of the sunlight that hits the Earth's atmosphere is scattered in different directions. This is what makes the sky blue during the day. However, this scattered light is not enough to illuminate the entire night side of the Earth.
Think of it like this: Imagine holding a flashlight in a dark room. The light from the flashlight directly illuminates only the area in front of it. The rest of the room remains dark.
That's why when the sun is up, we have daylight, and when the sun is down, we have night!
