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Because the phase of a substance is determined by how its molecules are configured, many physical properties of that substance will change abruptly as it goes from one state to another. In the recent paper, the researchers measured several telltale physical properties of water at temperatures between 0℃ and 100℃ under normal atmospheric conditions (meaning the water was a liquid). Surprisingly, they found a kink in properties such as the water's surface tension and its refractive index (a measure of how light travels through it) at around 50℃.

This gives water properties that, in many cases, break the trends observed for other simple liquids. For example, unlike most other substances, a fixed mass of water takes up more room as a solid (ice) than as a (liquid) because of the way it molecules form a specific regular structure. Another example is the surface tension of liquid water, which is roughly twice that of other non-polar, simpler, liquids.

Water is simple enough, but not too simple. This means that one possibility for explaining the apparent extra phase of water is that it behaves a little bit like a liquid crystal. The hydrogen bonds between molecules keep some order at low temperatures, but eventually could take a second, less-ordered liquid phase at higher temperatures. This could explain the kinks observed by the researchers in their data.