Place the yeast and warm water into a medium size bowl and stir to dissolve the yeast. Add the flour and stir vigorously with a wooden spoon or your hand for 2 to 3 minutes, until smooth, and dough is elastic. The starter will be thick and stretchy. It will become more elastic after it has risen.
Scrape the starter into a clear container with high sides, and cover with plastic wrap. Mark the height of the starter and the time on a piece of tape on the side of the container so you can see how much it rises.
At this point, you have two options. If you plan on making the dough the same day, let the sponge rise at room temperature until it has risen to the point where it just begins to indent on top. This may take 6 to 8 hours. It will triple in volume, and very small dents and folds will begin to appear in the top of the surface as it reaches its peak and begins to deflate. Use it before it sinks too much.
If you plan to make the dough the following day, let the sponge rise for 1 hour after mixing, then place it in the refrigerator and let it rise for at least 14 hours before taking it out to use. Be sure to compensate for the cold temperature of the starter by using warm water in the dough. Let it sit out, covered, until it reaches room temperature. This may take several hours.