Food colouring. It works best with several colours.
Dishwashing Detergent
Toothpick or pencil.
What to do
If you can, do this activity in a sink, so cleaning up is easier.
Place some milk in the saucer.
Place four separate drops of food colouring on the milk. Spread the drops out evenly. Use different colours if you can.
Watch the milk and colouring for a while. Does anything happen?
Smear some detergent on the tip of the toothpick.
Touch the toothpick to the centre of the milk.
You should find the colours jump away from the toothpick. They may start to swirl around and mix together.
Try touching the toothpick against a few other spots on the surface of the milk.
What's happening?
The milk moves when the detergent reduces the surface tension of the milk at the point where it touches. As it moves, it drags the colouring with it, producing the swirling effect.
In many liquids, the molecules of the liquid are attracted to each other. This attraction makes the surface of the liquid act like a stretched-out balloon skin. Every point on the surface of the liquid is under tension. In water, the tension is only very slight and it is fairly easy to break through the surface, but if you have ever done a "belly-flop" into a swimming pool, you have felt the effect of surface tension.
Different liquids have different amounts of surface tension. Milk is mostly made of water, so it has a similar surface tension. The surface tension of Mercury is more than six times as great as water. A mixture of milk and detergent (or soap) has much less surface tension than plain milk.
Normally, surface tension doesn't make things move around on the surface, because there is the same amount of force pulling in each direction and it cancels out. When you put a bit of detergent into the milk, the surface tension at that spot was reduced. When this happened, the force on the surface around it didn't cancel out any more, so the milk on the surface pulled away from that spot, pulling the food colouring with it. As it moved, it started currents in the milk which started mixing the milk and colouring together.
Applications
Another famous demonstration of surface tension is the "detergent-powered boat". If you put a plastic tag from a loaf of bread onto the surface of the water, then put a drop of detergent into the hole in the tag, the surface tension near the hole will be reduced and the tag will be pulled in the other direction. It's not really powered by detergent, but by the surface tension of the water.
There are some insects, called water skimmers, which are able to walk on water, by using surface tension. They land on the surface of the water, but are so light they don't break the surface tension. Then by flapping their wings, they can zip around on the surface.
The food colouring in the milk.
Adding a drop of food colouring start the food colouring moving.
