Triangle sushi is a rice ball, also known as onigiri, that’s shaped into the form of a triangle. Although onigiri can be made into various shapes, triangle-shaped onigiri is typically the most common.

Named for Plato, who theorized that the classical elements were made of these regular polyhedrons, three of the Platonic solids are composed of equilateral triangles: the tetrahedron, octahedron, and ...