Transcript of the actual theory:
Theory: sandlings NEED water, because they actually feed off investiture.
First: the experiments done by Khrissalla in White Sand 2 show that sand that has passed through the digestive system of a sandling has been stripped of the investiture-absorbing microorganism, making it similar to Darkside sand. This implies that sandlings eating white sand must be a common enough occurrence for post-digested sand to be possible to retrieve.
Second: while it’s known that sandlings will avoid areas where Dorim vines grow because of the large amounts of water, we also see various species attack and consume living, carbon-based, water-filled prey. So water must not be fatal to them, or else consuming a person whose body is made of 80% water would kill them. It must just dissolve carapace.
Third: Sandlings, like greatshells, do not obey the square-cube law - they should collapse under their own weight, but sandlings don’t have the benefit of decreased gravity or floaty spren bonds, which means there must be something else keeping them alive. What better way than pure investiture?
So what if they don’t consume larger prey for nourishment, but rather for a water reserve? Then they use that water, housed within their digestive tract, to activate the white sand that they consume, sparking the investiture into a kinetic state and absorbing it. The microflora coating the sand is then also consumed, and used to build the terken carapace. Because it is no longer invested, it appears black or grey, but it is still sensitive to water and starts to unbind when it comes into contact with it. Similarly, when a terken carapace touches kinetically invested sand, those same macromolecules naturally either spark or leech away the investiture, making Sand Mastery ineffective.
Sandlings then can be thought of as a sort of giant earthworm, eating dirt, absorbing necessary nutrients, then pooping out the stripped minerals. Also kind of like any number of birds that eat rocks, which they store in their gizzard and use to help digest food. Sandlings just have a water-gizzard.