Surface tension
On a fluid’s surface we can observe forces that keep the outmost molecules together, creating a kind of skin.
Some insects, such as water striders, can even walk on this tight skin.
Surface tension can also prevent water from infiltrating cloth to dissolve the dirt.
For that reason, detergents contain special substances to reduce the fluid’s surface tension”.
These are called surfactants or tensides. A water drop’s shape is mostly a result of these forces.