Water molecules are attracted to one another through a force called cohesion which creates delicate film across the top of the water known as surface tension. Surface tension can be disrupted so that water molecules no longer “stick” together. The soap on the tip of the leaf attempts to diffuse through the water by moving from the most concentrated area of soap to the least concentrated area. As the soap disperses, it breaks the surface tension behind the leaf , and it is pushed across the water. Once the soap has completed diffused through the water, the leaf boat will no longer move.
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