13. What is a sponge?

A batter or soft dough containing all of the water, but only part of
the flour and (usually) none of the salt. The starter (or,
conservatively, part of the starter) is mixed into it, and thereafter
it is incubated (at some temperature between freezing and heat death)
until it gets frothy, at which time the dough is completed with
additional flour, salt, and usually some kneading.

-Dick