So, in Animorphs, the kids gain the ability to morph into any animal they can touch. For the most part, the exact mechanics are hand-waved due to it being magic-like alien technology, but one in particular was not.
When they morph into anything, their mass also changes. They do not become a 150-pound mosquito, for example. And in book 12, The Reaction, Rachel "burps" a full-grown crocodile, all that mass coming from... somewhere.
Normally this wouldn't bother me too much; Applied Phlebotinum (TVTropes) doesn't need explanation. However, in book 18, The Decision, we get a direct look at at least one of the mechanics behind how the morphing power works:
When using a low-mass morph, the extra mass is extruded into Z-Space (a parallel dimension mostly used for faster-than-light communication and travel), then retrieved when the morpher returns to their natural form.
What I don't recall is whether or not it's ever explained where the extra mass from a high-mass morph comes from - such as when Rachel becomes an elephant, or when that crocodile was created.
The best I could find so far is an (unsourced) Word of God joke from TVTropes - on the ShapeshifterBaggage page:
Applegate explained once that it came from "Black holes, and pygmy hippos."
Is that really the only explanation out there, and is it a real quote?