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Toothpick Stars

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.

Objectives

  • Describe the relationship between capillary action, adhesion, cohesion, and surface tension.

  • Explain how capillary action is used by plants.

Materials

  • Per Student:
    5 wooden toothpicks, brand new and dry
    eyedropper
    small glass of water (can be shared between two students)
    plate (optional)

Key Questions

  • How does the water move to the ends of the toothpicks?
  • Why does the star expand?
  • Could you repeat this experiment with plastic toothpicks?

What To Do

  1. The students snap their 5 toothpicks half-way, so that they are hinged in the middle. Do not separate the two halves of the toothpick; each toothpick should be in a “V” shape.
  2. Arrange the toothpicks in a circle on a flat surface (or plate) with the arms of the toothpicks touching each other. Try to arrange the toothpicks as symmetrically as possible for the best result.
  3. Carefully squeeze a drop of water into the middle of the toothpicks. Make sure the drop touches the snapped part of each toothpick. You may need to add more water to get it right, but not so much that the toothpicks are floating!
  4. Wait several minutes and note what happens. The toothpicks will slowly move into the shape of an open 5-pointed star.

Extensions

  • How does water go from the soil upwards into the stems of plants?
  • Place a stalk of celery in water containing food colouring and follow the path of the dye.

Other Resources

Science World | Show Us Your Science | Magic Blooming Flowers