If you caught my post on puzzle jugs and fuddling cups, this should have been included, but I didn't know about it until today. Not quite as elegant as the Pythagoras cup, it functions on the same principle; pour enough liquid in and the siphon activates draining it out the bottom.
Here's a page with some more examples of glassware functioning this way (although, giving the alternate name of dribble cup, seems inaccurate, that, as commonly understood, was a much later prank invention as seen here). And if you are interested you can buy one here.
I know, I know, not exactly the elegant rules I intended this blog to be about, but so kooky, I thought I'd share. And it's Friday. :)