Why is the sky blue?

The Earth’s atmosphere contains tiny
molecules of gas and dust particles. Sunlight
entering the atmosphere hits these molecules
and dust particles. Colours with longer
wavelengths, like red and yellow, can pass
through the atmosphere without being
scattered by these molecules of gas and dust
particles. But the colour blue, with its shorter
wavelength, is scattered by the gas molecules
and the dust in the upper atmosphere. This is
why the sky appears blue.

Blue water
Water is actually colourless. However, large amounts of
water act like the sky and scatter blue light. This is why
seas, lakes and rivers usually appear blue.