pythagorean espresso cup

this is anpythagorean cup

It seems a simple mug, but there is a trick: if you fill it till a certain level, no problem, you can taste your drink; but if you are greedy and you want to fill it till the top... the mug rips off you, and it becomes completely empty.

 in Greece you can find lot of cups with somthing like a pin inside it.
this mug is even better: no pin inside, but it works too!! Can you guess how it works?

here video:

https://www.instagram.com/p/BmjT0ZHnUD_

if u drink out of this, please use bio compatible material