It is stated that water is extremely sensitive to THz radiation, absorbing big amounts of this radiation, this being a reason why skin measurements are safe to do using this range of frequencies (1) (2) (3)
But I don't find a good explanation as to why this is so.
The last link states that the reason behind the absorption of THz by water is "due to rotational transition of polar water molecules" but I don't understand this statement.
Water molecules vibrations comprise a range of frequencies in the THz range, so when radiation of a specific frequency hits a water molecule vibrating in that same frequency it causes them to "absorb" the radiation.
This still does not explain why they absorb the radiation and I don't understand the reason as to why.