"Ink" is the name given to a semi-transparent black oil-like substance found in many lifeforms. It's a type of matter that forms between clashing dimensions or universes.

In Spaceville, which is a 3D universe, a thin layer of ink covers the bodies of all 0-, 1-, and 2-dimensional beings. It is soluble in water and has to be reapplied after a lesser-dimensional creature interacts with water.

It must be noted that lesser-dimensional beings in Spaceville are technically still 3D; while ink greatly reduces their linear dimensions, it takes time to form and in that time allows their "missing" dimensions to be partially restored.