Parents sometimes have moments in which they are caught speechless by a realization about the world as their child sees it. In this case, the realization was this: her daughter had never known water as anything beyond something to drink or to rinse off with in the shower. Cas wanted to know what it was like to have so much water available that you could jump into it, sink into it, be in it. This required a well-considered answer.
From Water: Generation Mars, Book Three. Available now at https://www.amazon.com/Water-Generation-Mars-Book-Three/dp/1733731067
Image: Modified from https://commons.wikimedia.org/wiki/File:A_girl_in_a_swimming_pool_-_underwater.jpg (Reg Mckenna from UK, https://creativecommons.org/licenses/by/2.0/ )