In this activity, students use capillary action to move broken toothpicks into a star shape without touching them.
The toothpicks the students are using are made of dry wood. When the water is placed in the middle of the closed star formation, the wood begins absorbing the water, causing the wood to expand.
But how does the wood absorb water?
The adhesive force between the water and the wooden toothpick is stronger than the cohesive forces inside the water itself. The adhesive force pulls the water molecules into the narrow spaces within the wood. Cohesion (forces between water molecules) ensures that other water molecules trail behind. This process is called capillary action. The result is that the water travels to the tips of the broken toothpicks.
As the wood absorbs more of the water, the bent wood fibres expand and straighten out. Each toothpick end pushes against the others. As the toothpicks straighten and push against each other, the inside of the star opens up.
Capillary action can also be seen in plants. Plants contain many vein-like tubes that carry water from the roots upwards to the highest leaves via capillary action.
This is a recommended post-visit activity to Science World.