Variational principles, invented in analytical mechanics, has become a tool in various fields of physics, engineering and applied mathematics. Very often, in an introductory course of analytical mechanics, there is not time for the discussion of variational principles in other disciplines. Perfect fluids represent a simple system for which the usefulness of Hamilton's principle can be easily demonstrated. Moreover, the physical notion of Lagrange multipliers and the role of constraints can readily be discussed for such a system. From the educational point of view this example can also be used as an introduction to the discussion of variational principles in continuum mechanics.