# What 3d shape has only 1 side?

There are many 3d figures, but can you figure out what 3d figure (with a completely flat surface) has only 1 side?

• Can you elaborate on what you mean by "with completely flat surfaces"?  Why do you use the plural (surfaces) for a figure that has only one side — what's the distinction? – Peregrine Rook Jun 6 '16 at 18:27
• A single-track audio tape cassette (which doesn't have a Side B). :) – KeyboardWielder Jun 6 '16 at 18:39
• If you are wondering why this question is garnering downvotes, it is because this is really just a trivia question, where the answer is even fairly common knowledge. This does not really fit on this site as the purpose is to create puzzles. In the future try to refrain from asking questions such as this one to avoid downvotes – Gordon Allocman Jun 6 '16 at 18:54
• @KeyboardWielder I'm sorry, but this is geometric stuff, and CDs aren't completely flat. – Xxoplechic Jun 7 '16 at 1:07
• I think what OP means by "completely flat surface" is that the figure has no volume, not that it's 2-dimensional. – paste Jun 7 '16 at 2:35

Seems a bit too easy but is it:

• That has two sides doesn't it? – Beastly Gerbil Jun 6 '16 at 18:21
• @BeastlyGerbil:  No, it doesn't.  But if it qualifies as having "completely flat surfaces", I don't understand what the OP means by those words. – Peregrine Rook Jun 6 '16 at 18:25
• @PeregrineRook I assume that was to rule out a solid sphere – NeedAName Jun 6 '16 at 18:26
• Yeah it depends on what he means by completely flat surfaces. Honestly I hope there is a more interesting answer than this – Gordon Allocman Jun 6 '16 at 18:31
• @PeregrineRook If it had any thickness that would mean it technically has 2 or more sides. – Xxoplechic Jun 6 '16 at 18:41

A Klein Bottle (wikipedia article)

• Yes, that works! I forgot about that (I read it in a book). – Xxoplechic Jun 6 '16 at 19:18
• That was my first thought when I saw Gordon's answer. – paste Jun 7 '16 at 2:34

I don't think it's anything like an actual 3D shape

Is it