This is a 3D printed Pythagorean cup, otherwise known as a greedy cup, where if one pours far too much water or wine or whatever your greedy heart desires, all the contents in the cup will leak out through the bottom.
This is based on the design by “jsteuben” on Thingiverse (https://www.thingiverse.com/thing:123252). The siphoning effect kicks in when the water level is above the internal “tube” printed and hidden into the walls of the cup.
I printed another cup based on a more conventional design as well, but due to the wrong settings given when I prepared the gcode file, the cup was rather leaky when the water level was low. This design by “MonzaMakers” has a protruding siphon tube. (https://www.thingiverse.com/thing:562790)
Explaining how the siphon works is easier with the second cup. When the water level is lower than the highest point in the siphoning tube, it remains in the cup. When it exceeds the highest point of the tube, water begins to flow down the part of the tube leading to the opening at the bottom of the cup. The falling water column creates a suction effect and continuously draws the rest of the water in, until the cup is dry.