How Does It Work? Surface tension gives water some amazing properties. The cohesion and surface tension of water becomes apparent when the drops of water you add to the penny reach the penny’s edge. Once the water has reached the edge, you begin to see a bubble or dome of water forming on top of the penny as you can see here in the video. The bubble shape is a result of the water molecules clinging to one. I was able to get 30 drops on a penny. Let me know how many you were able to do?