  A paperclip can float on water. Drops of mercury refuse to spread on a
surface. These capillary phenomena are macroscopic manifestations of molecular
interactions, and can be explained in terms of surface tension. For students,
the concept of surface tension is quite challenging since the microscopic
intuition is often in conflict with the common macroscopic interpretations. In
this paper we address a number of conceptual questions that are often
encountered when teaching capillarity. By answering these questions we provide
a perspective that reconciles the macroscopic viewpoints, from thermodynamics
or fluid mechanics, and the microscopic perspective from statistical physics.
