In the surface of water it exist a very known phenomenon - superficial tension. Due to superficial tension of water we don't obtain tubes without adding detergent to it. When we add detergent to water, the superficial tension decreases, facilitating the formation of soap tubes. The resulting soap tubes don't last for long time, because the water film evaporates very fast. Using glycerin, the speed of water evaporation is slower and the tubes last much more time. When we have rainy days the soap tubes will last even more time. Try it!