A friend sends this:
In 1882, Klein imagined sewing two Möbius Loops together to create a single sided bottle with no boundary. Its inside is its outside, it contains itself, and you can almost do it yourself… but not quite because you’ll need a fourth dimension to get the surface to pass through itself without making a hole.
Well, if you can’t make one, you can always buy one, at Acme, of course, http://www.kleinbottle.com/. Yep, no doubt about it: Your Acme's Klein Bottle is a real Riemannian manifold, just waiting for you to define a Euclidean metric at every point.
Their ad says:
Need a zero-volume bottle?
Searching for a one-sided surface?
Want the ultimate in non-orientability?
Get an ACME KLEIN BOTTLE!