[tempodox]

While this is true mathematically, there is at least one project that emulates Klein bottles in 3D quite credibly:

http://www.kleinbottle.com

Edit: You can even put in some liquid (the “hose” continues through the crossing). The fun part is getting it out again :)

[emmelaich]

Hey that's the famous Clifford Stoll of Cuckoo's Egg fame.

Glad he is not famous only for that book. As fine as it is.

[mcherm]

There are lots of projects that do this (implement a Klein bottle with a crossing).

[dancek]

Even my dad has knit several Klein bottle hats for fun, inspired by one of those projects. I got one as a Christmas present.