  It is well-known that Lagrange's four-square theorem, stating that every
natural number may be written as the sum of four squares, may be proved using
methods from the classical theory of modular forms and theta functions. We
revisit this proof. In doing so, we concentrate on geometry and thereby avoid
some of the tricky analysis that is often encountered. Guided by projective
differential geometry we find a new route to Lagrange's theorem.
