24 times the same tetrahedron for a cube
This is a dissection of the cube in equal non regular tetrahedra. The piece is defined by its four vertices: two vertices compose an edge of the cube. A third is the center of a square, face of the cube, and the last one is the center of the cube. There are four edges in each direction of the 3D space, so 12 in total. Each edge belongs to two faces, hence two tetrahedra so there are 24 tetrahedra.