Water is not just a wet substance. On a picosecond scale, it resembles a dance floor where molecules continuously break and form bonds with each other. Usually, this dance is described by one main movement — collective relaxation, as if the whole crowd simultaneously slows down after the music stops. But at very high frequencies, around one terahertz (that's a trillion oscillations per second), a second, faster rhythm emerges.

The authors measured how water absorbs radiation in the range from 0.14 to 1.21 terahertz. The standard description proved insufficient: they had to add a contribution resembling the behavior of electrons in disordered conductors (the so-called Drude–Smith model). This contribution strengthens when heated from 20 to 50 degrees and weakens if ordinary water H₂O is replaced by heavy water D₂O. Computer simulations showed that the culprit is 'rule-breaking' molecules: those where the number of donated and accepted hydrogen bonds doesn't match. As temperature rises, such molecules become more numerous.

But the most interesting part is the nature of this signal. Calculations showed that it arises not only from nuclear motion but also from instantaneous electron redistribution. When the hydrogen network momentarily loses balance, the electron cloud of the molecule shifts, and this generates an additional electrical response. Thus, at terahertz frequencies, water reveals itself as a mix of nuclear dance and electronic echo.