  We give a pedagogical review of the recently-introduced notion of a "scalar
product" between Feynman integrals and how it helps us understand the analytic
structure of the perturbative S-matrix. (This article is a contribution to the
proceedings of the workshop "MathemAmplitudes 2019: Intersection Theory and
Feynman Integrals" held in Padova, Italy on 18-20 December 2019.)
