If this would be my desk, where should i put the coffecup?
Obviously you would use a Klein bottle.
Klein's bottle: the reason why the topologist died in the desert.
They tried to fill it with water, but every time they poured some in, it was out.
@yours_truly @mina @number6 it is the only drinking vessel where I think it is not only appropriate, but also reasonable to use a straw not only to drink, but also to fill it