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“The chemical composition of “water” is well-known around the world; even children can usually easily state its molecular formula. Yet, “water” is also the strangest chemical liquid! Currently, a group of scientific observers is studying another strange property of water: aside from the state of flow of water coming from our household faucets (phase, which can also be referred to as \"state\"), abnormal changes in water’s density show that at low temperatures, water indeed exists in two \"liquid phases\". (For ordinary chemical substances, there should be only one state per phase – for example, the solid phase is a substance in a single solid state, while the liquid phase is a substance in a single fluid state.) Water possesses several \"unique properties\" that other liquids do not have; for example, it can absorb a large amount of heat energy without its temperature rising too much ； As it cools down, its compressibility increases, which is the exact opposite of what happens with ordinary liquids. And the most peculiar thing is that as the temperature drops, the density of water increases up to 4 degrees Celsius, and then it gradually decreases again. This is quite different from the typical property of liquids, where density decreases steadily as temperature drops. For such a phenomenon that even middle school students are aware of, few people have ever studied its true causes; it was only recently that some progress was made in this regard. Scientists suspect that this complex phenomenon is caused by “dual traits”” ； They first proposed that water does indeed exist in two different \"liquid phases\" at low temperatures: one is a high-density (HDL) liquid, and the other is a low-density (LDL) liquid. These two liquid phases will not be separated, because they can only exist at temperatures below 45 degrees Celsius, at which point water is almost frozen. On May 21st, in the journal Physical Review Letters, a team of German researchers published findings suggesting they may have observed water in a high-density liquid state. They used X-rays to irradiate an ice cube placed on a silicon dioxide plate at minus seventeen degrees Celsius, while measuring the parameters reflected back, and found that there was a very thin layer of water (about only five molecules thick) at the interface between the ice cube and the silicon dioxide plate. And the density of this layer of water is 17% higher than that of room-temperature water. They also observed that in this extremely thin interface layer, water molecules are unable to move around rapidly at this temperature; for a transition (mixing) to occur between the high-density (HDL) and low-density (LDL) water phases, it is necessary to be at an effective temperature of minus seventeen degrees Celsius, or around minus forty-five degrees Celsius. In other words, water originally exists in different forms, and they do not mix easily with one another. The findings of Engemann S, Reichert H, and Dosch in this research have taken theoretical studies on the existence of two phases of liquid water a significant step forward. This seemingly very ordinary phenomenon of ultra-fine layers in water, together with an understanding of the properties of high-density and low-density liquid water, will enable us to gain a broader perspective by looking inside water, thus allowing us to understand more deeply the important role that water plays on Earth. This also leads us to an alternative thought: are seawater and fresh water two different phases of liquid water? When the two are mixed, is there also an imperceptible boundary? That’s actually a pretty interesting idea!