Surface tension:
Surface tension in water is caused by the strong attraction between water molecules. The whole surface of water is attracted downward. The net effect is an inward force at its surface that causes the liquid to behave as if its surface were covered with a stretched elastic membrane.
I first thought that there is a surface force pulling them horizontally. Just like an elastic membrane that is under tension has to feel a stretching force. Even though surface tensions caused the surface to act like an elastic membrane under tension, it is not because of the stretching of the membrane. There is, but it is not the point of surface tension.
The surface wants to be minimized (this is a bit more abstract)
Penguins
Penguins have funny group behaviour to minimize heat loss. This behaviour is quite alike surface tension.
Surface tension is similar to a bunch of penguins keeping each other warm on the south pole:
A writing place. 