Hold one of the translucent solids in the light. You should see a glowing two-dimensional shape that changes as you move the solid around. The glowing shape is called a "cross-section" of the solid.
Can you find a hexagonal cross-section of the cube? Which two of these solids have a square cross-section? What surprising shape can you find in the triangular pyramid? Which two solids have elliptical cross-sections?