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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!

Ages 4-8 0-1 hours Education 8/10

Materials

  • Cotton Wool
  • Food Colouring
  • Full-fat Milk
  • Paper Plates
  • Washing-up Liquid

Illustrated Steps

1

Pour the Milk

Pour a shallow layer of full-fat milk into a paper plate — about 1 cm deep, covering the bottom completely.

2

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.

3

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.

4

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!

How to Set It Up

Step 1: Pour the Milk

Pour a shallow layer of full-fat milk into a paper plate or flat dish. The milk should cover the bottom completely but stay quite shallow — about 1 cm deep is ideal.

Step 2: Add Colour Drops

Drop 4–5 drops of food colouring in different colours around the surface of the milk. Try red, blue, green, and yellow for maximum drama. Do not stir — just let them sit.

Step 3: Coat Cotton Wool in Soap

Dip a small piece of cotton wool into washing-up liquid to coat it. Alternatively, simply drop one small drop of washing-up liquid directly into the centre of the milk.

Step 4: Touch and Watch

Gently touch the soapy cotton wool to the surface of the milk. The colours will explode outward in swirling, firework-like patterns! Touch different spots to keep the reaction going.

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.