Geometry which incorporates food!!! (You don't actually have to use real sandwiches)
Students must use a triangle congruence theorem (either side-side-side, side-angle-side, angle-side-angle, angle-angle-side, or hypotenuse leg) to prove that when you cut a sandwich in half diagonally, the two resulting triangles are congruent and also that when you cut it into quarters, all four triangles are congruent!
Examples included for students so they can see what to do!
Make sure you download the preview because my example proofs are not particularly formal, although you could ask that your students are!