From wiki: &quot;Whereas a Möbius strip is a surface with boundary, a Klein bottle has no boundary (for comparison, a sphere is an orientable surface with no boundary).&quot; (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Klein_bottle" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Klein_bottle</a>)