If you spill water, you will see that the water does not spread out into a thin film but contracts into droplets. This is because water wants to make its surface as small as possible. We also call this a high surface tension.
If you add soap, you lower the surface tension. This way, you can stretch water into a thin film. With water and soap, you can make large bubbles, or even a large soap tube in which you can stand!
Surface tension with detergent
By adding detergent to your washing machine, you lower the surface tension of water. This allows the water to penetrate deep into the fibres of your clothes. No more stains!
