A
nice pointless abstract concept. This is like a
mobius strip, in that it only has one side, but it exists in 4
dimensions. Otherwise known as a
klein bottle (thanks
chris-o). A 3-d representation of this looks like a
vase whose head twists around to the side, passes through the side of the vase and joins up with the base of the vase to continue as the wall, but it exists in 4-d so it apparently can do this
without ever touching itself.
It's also very hard to explain.