Proud of their theoretical acumen, shunned by an uncaring reality, a group of glassblowing mathers are selling Klein Bottles: the volumetric versions of Möbius strips. Neat huh? So... where does the air go when you try to fill it with a liquid? Look at the mug again: it's like one of those magic pitchers that looks like it's pouring but nothing comes out. How many rounds of adding a little water, tilting the mug over, adding a little water, tilting the mug over do you think it took to get that picture?
Boy, that's a drinking experience I could miss.