No one is quite sure, especially since they seem to violate the laws of conservation of mass and energy being able to create and dissipate mass at will as well as generate energy for the respective Tank. It's theorized that the extract is actually a soup of tiny machines which can remotely pull from an unspecified hybrid mass/energy reserve which is actively applied to a target either as extra organic mass or by creating a filling material such as air or helium. It's also able to produce modified cells that tap into this unspecified source to produce things like milk. All of this reverted and returned to it's source when the Tenk wills it. Other theories exist but none come as close to explaining the seemingly infinite nature of the growth. Interestingly enough Sara wrote several papers on similar topics regarding remote mass/energy access and transfer so this is one of the big things she focuses on for her research since the implications are massive.