Surface Tension with Toothpicks

Do you remember our lolly stick races? They are a great way to explore surface tension, but this activity demonstrating surface tension with toothpicks is even easier!

The toothpicks start off in a triangle shape and then fly apart when a little washing-up liquid is added.

toothpicks floating on a bowl of water, washing up liquid and pond skaters on water — Surface tension with toothpicks activity

Surface tension activity with toothpicks

You’ll need

Cocktail sticks or toothpicks
A clean bowl
Washing up liquid /dish soap
A large bowl

Instructions

Fill the bowl with water and let it settle for a few minutes.
Carefully place the cocktail sticks on the top. Try and get them to form a triangle.

3 cocktail sticks lying on a bowl of water in a triangle shape — toothpicks in a triangle ready for a surface tension activity

Add a drop of washing-up liquid, and watch what happens.

cocktail sticks moving apart on a bowl of water in a surface tension activity

Why does it work? Surface tension explained!

This activity is a very simple demonstration of surface tension. When you have a container full of water, the water molecules below the surface are pulled together equally in all directions, but those on top are pulled together more tightly, as they don’t have water molecules above them. This draws them together to form a kind of ‘skin’ that we call surface tension. When the washing-up liquid is added, it disrupts the arrangement of the water molecules, decreasing the surface tension inside the triangle of toothpicks.

Water molecules move from areas of low surface tension to high surface tension, and so the sticks move.

Don’t worry if that explanation seems too much for your child. I don’t always explain the full scientific principles to my children, but watching and taking part in activities like these really does encourage children to start to question why things happen, and the concepts will start to make more sense to them as time goes on.