What colour is water?

We usually think of water as a colourless liquid, but it’s not true. When white light is shone through a long column of the stuff, the liquid is faintly blueish. That’s because the water mops up the longer, redder wavelengths of light, leaving the bluer wavelengths unscathed. As with so many of its properties, however, water does this in a bizarre way, with the light energy being absorbed by its V-shaped H2O molecules, which start to vibrate as a result.