Tetrahedron is one of the 5 platonic solids that has 4 vertices, 4 edges and 4 faces. 

It is simple but many of its properties are a bit unexpected. Unlike other simple shapes like cube and pyramid shape, tetrahedra doesn't tile, you cannot fill up space with this shape alone. 

This toy is made of interconnected regular tetrahedrons that you can rotate at each joint. It will allow you to play with this interesting shape and see what you can build with it.