Justice cup is a small siphon device with a long history. As long as it does not exceed the limited water level, it can normally fill tea and pour wine, but once it is too full, all the liquid inside will flow away:

the principle and structure of the justice cup are actually not complicated, as long as a suitable siphon is added to the ordinary cup, a justice cup can be made.

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the simplest version of the DIY justice cup can be made from disposable plastic cups, straws and hot melt glue and can be made in a few minutes.

first use an awl to make a hole in the bottom of the plastic cup that allows the straw to pass through (this plastic cup is very soft, and other tools such as scissors can actually penetrate it):

pass the long side through the hole in the bottom of the cup, let the inverted U-shaped part stand in the middle of the cup, and the short edge opening is close to the bottom of the cup. Next, seal the gap around the straw with hot melt glue from the outside of the bottom of the cup:

then cut off the excess straw on the outside of the cup so that the cup can stand on the table. The structure of the

cup is shown below. The length of the inverted U-shaped part determines the highest water level. The water level will be relatively low with a single straw. If you want to increase the height, connect a section of the straw on the short side and seal it with hot melt glue to avoid leakage.

actual effect (accelerated):

this version has no problem in demonstrating the siphon effect, the main problem is that the siphon is very simple, and the siphon stands in the middle of the cup, and the cup is transparent. It looks obvious.

in order to make the siphon look a little more hidden, I also made another version of the justice cup, and the effect is as follows (still accelerated):

this time the idea is that the siphon goes from the outside and hides most of it into the handle of the cup. I found a plastic cup with a hollow handle (yes, the handle is actually used for toothbrushes) and made a hole in the lower part of the cup wall facing the handle.

the schematic diagram is as follows:

in this way, there is only an opening to pick up the straw inside the cup, which will look more like an ordinary cup.

of course, there is still a big gap between beauty and fair cups made of ceramic or glass.... If you are interested, you can also try to do it yourself, or put forward more improvement plans.