Patrickg - topologically speaking - doesn't the rim of a wheel have two sides - the inside and the outside?
A Mobius strip has one side ...
(OK, OK, so it's not strictly speaking a polygon - but it is interesting!!)
Patrick
P.S.
An Engineer, a Physicist and a Mathematician were each placed in a room with a wooden table, a chair and a bucket of water, told to sit on the chair and the table set on fire.
The Engineer leapt up, ran to the bucket, picked it up and threw the water at the table, putting the fire out.
The Physicist got up, made some measurements, and then threw exactly the right amount of water over the fire so that it was put out, vapourising the water in the process, so the table was left dry.
The Mathematician got up, took out his notebook, looked at the burning table, looked at the bucket, made some calculations and said "Ah! A solution exists!" and sat down again, letting the table burn to a cinder.
A little while later the experiment was repeated, this time with the bucket of water placed on the table (they replaced the Mathematician's table).
The Engineer and Physicist did exactly the same as before.
The Mathematician looked at the table and bucket, then took the bucket off the table and put it on the floor, thereby reducing the problem to a previously solved one. He then sat down and let the table burn to a cinder again.