  Feynman integral computations in theoretical high energy particle physics
frequently involve square roots in the kinematic variables. Physicists often
want to solve Feynman integrals in terms of multiple polylogarithms. One way to
obtain a solution in terms of these functions is to rationalize all occurring
square roots by a suitable variable change. In this paper, we give a rigorous
definition of rationalizability for square roots of ratios of polynomials. We
show that the problem of deciding whether a single square root is
rationalizable can be reformulated in geometrical terms. Using this approach,
we give easy criteria to decide rationalizability in most cases of square roots
in one and two variables. We also give partial results and strategies to prove
or disprove rationalizability of sets of square roots. We apply the results to
many examples from actual computations in high energy particle physics.
