Human: I've always wondered: Why is the sky blue? Assistant: 
The sky is blue because of Rayleigh scattering. When sunlight hits the atmosphere, it gets scattered in all directions. Some of the light is scattered back toward the Earth's surface, and some is scattered into space. The light that is scattered into space has a longer wavelength than the light that is scattered back toward the Earth's surface. This means that the light that is scattered into space is more likely to be scattered again by molecules in the atmosphere, which causes it to be scattered even more. This process repeats itself until the light reaches the Earth's surface.