Surface Tension

Demonstrating surface tension with a loop of thread. A loop of thread is floating on the surface of a bowl of water. The surface tension inside and outside of the loop are the same, and the loop is limp. But what happens if I put a drop of soap inside the loop? The surface tension inside the loop decreases, but the surface tension outside is initially unchanged. There's an unbalanced force pulling on the outside edges of the loop, and it springs into a circle! Eventually the soap diffuses over the entire surface of the water, and we return to having equal surface tensions inside and outside the loop.