Here's why:

* Sunlight: Sunlight is made up of all colors of the rainbow.

* Scattering: When sunlight enters the Earth's atmosphere, it collides with tiny particles like nitrogen and oxygen molecules. These particles scatter the light in all directions.

* Blue Scattering: Blue light has a shorter wavelength than other colors, so it gets scattered more strongly than other colors. This scattered blue light is what we see as the blue sky.

The sky can appear other colors at times:

* Sunrise and Sunset: During sunrise and sunset, the sunlight has to travel through more of the atmosphere. This causes the blue light to scatter away, and we see more of the longer wavelengths like red and orange.

* Clouds: Clouds are made up of water droplets or ice crystals, which reflect all colors of light equally. This is why clouds often appear white or gray.

So, while the sky is mostly blue, it can change color depending on the time of day and atmospheric conditions.