Tie-dye milk

Watch your milk turn multi-coloured!

It is all to do with surface tension.  Look closer next time you have a glass of milk. Notice that the surface of the milk looks like it has a very thin skin.  This is called surface tension. You'll notice it on water too. It's what pulls water into little droplets on a smooth surface.

When you add the food colouring it doesn't break that invisible skin, it just sits there. Some insects are really good at sitting on this skin without breaking it. Pond skaters do it really well!  Washing-up liquid breaks the surface tension.  This allows the food colouring to flow freely through the milk. This experiment reveals some important things about liquids, but we mostly like it because it looks so cool!