Add one drop of red, green, yellow and blue food colouring to the centre.
Put some liquid soap onto the cotton bud.
Place the soap end of the cotton bud to the centre of the colour.
A rainbow affect is created because the soap is an emulsifer – which means it is able to mix two liquids together that normally don’t mix together. When we add the food colouring to the milk, it does not mix.
This is becuase the water in the food colouring does not mix with the fat in the milk. When you add soap, it helps the food colouring molecules disperse within the milk molecules, This is why we use washing up liquid. The soap helps grease on dirty dishes mix with water which makes it easier to clean.