Amy Scherber's Sponge Starter

Mix all the ingredients together in a medium bowl and stir vigorously with
a wooden spoon for 2 to 3 minutes, until a smooth, somewhat elastic batter
has formed. The batter will be very stiff; it gets softer and more elastic
after it has proofed. You may find it easier to mix the sponge using
electric mixer, with a paddle or a dough hook, on medium speed for 1 to 2
minutes. Scrape the sponge into a 2-quart clear plastic container and cover
with plastic wrap. At this point you have two options:

If you plan to make your dough later that same day, let the sponge rest at
room temperature until it has risen to the point where it just begins to
collapse. This may take from 6 to 8 hours, depending on the temperature of
the sponge, the temperature of the room, and the strength of the yeast. The
sponge will triple in volume and small dents and folds will begin to appear
in the top as it reaches its peak and then begins to deflate. The sponge is
now in perfect condition to be used in a dough. It’s best if you have
already weighed or measured out all of your other recipe ingredients before
the sponge reaches this point so you can use it before it collapses too
much.

If you’re not planning to make your dough until the next day or the day
after, put the covered sponge in the refrigerator and let it rise there for
at least 14 hours before taking it out to use in a recipe. Be sure to
compensate for the cold temperature of the starter by using warm water (85
to 90 degrees) in the dough instead of the cool water specified in the
recipe. Or let the starter sit out, covered, until it reaches room
temperature (this may take several hours)-but don’t let it collapse to much
before you use it.