Why do only some shapes tessellate?

tessellations are designs that are based on a shape that regularly tiles smoothly, such as squares or hexagons. Geometrically, this guarantees that all the space is accounted for, and that the shapes should fit together ( though not necessarily smoothly). If you take a square or hexagon (or any other regular shape that fits together by itself) and cut out parts of it using scissors, then attach the cut out parts on the opposite edge of the square from which they were removed, you should end up with a working tessellation.