Indoor activity
Milk Soap Fireworks
Watch an explosion of colour swirl across milk when you add a drop of soap — a dazzling display of surface tension science!
Materials
- Cotton Wool
- Food Colouring
- Full-fat Milk
- Paper Plates
- Washing-up Liquid
Illustrated Steps
Pour the Milk
Pour a shallow layer of full-fat milk into a paper plate — about 1 cm deep, covering the bottom completely.
Add Colour Drops
Drop 4–5 drops of different food colouring colours around the surface of the milk. Don't stir — just let them sit.
Coat Cotton Wool in Soap
Dip a small piece of cotton wool into washing-up liquid to coat it, or drop one small drop of soap directly onto the milk.
Touch and Watch
Gently touch the soapy cotton wool to the milk surface. Colours explode outward in swirling firework patterns!
What You’ll Create
A spectacular science display where food colouring erupts and swirls across the surface of milk the moment you touch it with washing-up liquid. It looks like tie-dye fireworks — and takes under five minutes to set up!
Have fun!
Try touching the soap to different parts of the milk — near the colours, at the centre, near the edge. Does it make a difference? What happens if you use skimmed milk? Try adding more colours for a rainbow explosion!
Why It’s Amazing
Milk contains fat molecules held together by surface tension. Washing-up liquid is a surfactant — it breaks surface tension by bonding to fat molecules. As the soap races outward to find fat, it drags the food colouring with it, creating beautiful swirling patterns. The reaction stops when all the surface tension is broken!
Pro Tips
💡 Full-fat milk gives the most dramatic reaction — higher fat content means more surface tension to break. 💡 Work quickly and photograph the swirls before the colours mix to muddy brown. 💡 Try double cream for an even bigger explosion — the fat content is even higher.