Bilateral Symmetry Meets Botched Schooling

(I am organizing my area of the store when a customer and her young daughter come in.)

Girl: “Mommy, why can’t I marry my cousin?”

Mother: “Because you’re too closely blood-related.”

Girl: “What’s ‘blood-related’?”

Mother: “It means if you had a baby together, it might come out with a deformity. You know… like having two arms.”