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 an 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 6 to 8 hours, depending on the temperature of the room and the strength of the yeast. The sponge will triple in volume and small dents 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 weighted 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 degrees to 90 degrees F) 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 too much before you use it.