Squaring the circle
The
quadrature of the circle, better known as
squaring the circle, is a classical problem of
mathematics, or more specifically, of
geometry. The problem is to construct, using only
rulerandcompass constructions, a
square with the same area as a given
circle. The problem dates back to the invention of geometry, and occupied mathematicians for centuries. It was not until
1882 that the impossibility was proven rigorously, though even the ancient geometers had a very good practical and intuitive grasp of its intractability.

The square and circle have the same area 
It should be noted that it is the limitation to just ruler and compass that makes the problem difficult. If other simple instruments, for example something which can draw an Archimedean spiral, are allowed then it is not difficult to draw a square and circle of equal area.
A solution demands construction of the number , and the impossibility of this undertaking follows from the fact that is a transcendental number, i.e. it is nonalgebraic, and only algebraic numbers may be constructed with ruler and compasses alone. The transcendence of pi was proven by Lindemann.
If you solve the problem of the quadrature of the circle, this means you have also found an algebraic value of π  this is impossible. This does not imply that it is impossible to construct a square with an area very close to that of a given circle.
The mathematical proof that the quadrature of the circle is impossible has not hindered many "free spirits" to invest years in this problem anyway.
The futility of undertaking exercises aimed at finding the quadrature of the circle has brought this term into use in totally unrelated contexts, where it is simply used to mean "a hopeless/meaningless/vain undertaking".