Sponge is a mixture of yeast, flour, and water that, when allowed to sit, bubbles and ferments; used in bread, it imparts a tangy flavor.
In the bowl of an electric mixer fitted with the paddle attachment, combine 1/4 cup warm water and the yeast. Let stand until foamy, about 10 minutes.
Add remaining 1 1/4 cups warm water and the flour; mix on low speed, 2 minutes; the sponge should feel like a very wet dough.
Place sponge in a lightly oiled bowl. Cover with plastic wrap, and let stand at room temperature for 24 hours. Store sponge, refrigerated, up to 1 week, or freeze in plastic for up to 3 months. Bring sponge to room temperature before using.