Sunlight looks white, but it is really made of all the colours of the rainbow mixed together, each with its own wavelength. As this light travels down through the atmosphere, it keeps bumping into tiny air molecules that are far smaller than the wavelength of visible light. Such small particles do not simply block the light, instead they redirect it in random directions, a behaviour named Rayleigh scattering after the scientist who studied it. The key point is that this scattering effect is very sensitive to wavelength, shorter wavelengths get scattered far more than longer ones, following roughly an inverse fourth power relationship with wavelength. Since blue light has a shorter wavelength than green, yellow, orange or red light, it gets thrown around the sky in every direction much more than the longer wavelength colours, which mostly travel straight through. Looking at any part of the sky away from the Sun, we mainly catch this scattered blue light entering our eyes, which is why the sky looks blue on a clear day.