  We present a conclusive answer to Bertrand's paradox, a long standing open
issue in the basic physical interpretation of probability. The paradox deals
with the existence of mutually inconsistent results when looking for the
probability that a chord, drawn at random in a circle, is longer than the side
of an inscribed equilateral triangle. We obtain a unique solution by
substituting chord drawing with the throwing of a straw of finite length L on a
circle of radius R, thus providing a satisfactory operative definition of the
associated experiment. The obtained probability turns out to be a function of
the ratio L/R, as intuitively expected.
