Try thinking of a single-sided figure. After some thought you might arrive at a sphere. Well there are others and they will open your mind. Take a look at a Mobius band. You can make this by taking a narrow piece of paper and rotating one end 180 degrees, then attaching it to the other end of the paper. This will result in a single-sided figure. There is no up or down, this side or that, the form is continuous.
Example, mobius band
Now you are ready for expansion, klein bottle