:smg:
I think you could be spot on with the spa water thing, Ivo!
I was watching a tv programme about vile sulphurous waters and the taking of them recently - there was a sort of spout sticking out of a wall, you pressed a button, and some of the water would splurt out of the tap.
The shape of this mug would take into account having to put it under the sticking-out tap to collect some of the splurting water. Accurate measurement is irrelevant, all you need is some of it.