In this make and take, students move their boats by varying the water's surface tension with soap.
The attraction of water molecules to each other (cohesion) can be weakened by the addition of soap/detergent. The attraction between water particles at the water's surface produces surface tension. The soap reduces the surface tension of the water around it.
You can use this effect to make a boat powered by surface tension. The placement of the soap in the small notch at the back of the boat means that the only way for the soap to disperse is by moving out the back, causing the boat to move across the water. The boat moves by being pulled forward by water molecules rather than being pushed by soap molecules. Since the surface tension is greater at the front of the boat than at the rear, it pulls the boat forward.
Once the soap has spread out across the surface of the water, the boat will stop moving forwards.