An impossible object is a picture of an object that looks three-dimensional but cannot be a two-dimensional projection of a real 3D object. It is a type of optical illusion. One of the best descriptions I have read about them was this one: "impossible objects are reasonable parts put together in un-reasonable ways".
And this is why they work: we interpret them as a two dimensional projection of a three dimensional object. So, there are two possibilities to make them: