In the north, jiaozi can be said to be an indispensable food on the table of every household during the festive season. Many people will eat jiaozi, but many people will not be jiaozi, let alone the mystery of jiaozi. There are several folk sayings about cooking jiaozi: "Boil the skin first, then the stuffing", "Cover the pot to cook the stuffing, cover the pot to cook the skin". These words are very reasonable.

As many northerners know, it takes three times of cold water to cook jiaozi in the north. What are the physical principles involved?

As we all know, if you want to cook jiaozi, you have to cook the stuffing in jiaozi. Although the dumpling stuffing can be cooked all the time, the dumpling skin outside is rotten, which is why cold water is needed.

It's kind of like fried food. If the temperature is too high, the outside will soon turn brown or even black, but the inside is not ripe yet. Therefore, if you want to cook the food thoroughly, you can't be too anxious. This requires a process, so the temperature should not be too high.

When the water boils, jiaozi will float. At this time, jiaozi was not ripe, but the skin was ripe and the dumpling stuffing was not ripe. If the water in the pot continues to boil violently, then jiaozi will boil excessively in the water, which will easily make the dumpling skin burst. At this time, cold water can be added to make the water still boil, but not too violently or even boiling, so that the swollen jiaozi can shrink back to prevent bursting and continue to heat the stuffing inside. Thermal expansion and cold contraction in physics.

This method of expanding with heat and contracting with cold can not only make jiaozi completely cooked, but also make the dumpling skin particularly delicious, so as not to be soft and tasteless. In addition to adding cold water, you can also achieve the same effect by opening the lid and controlling the heat.