With the Sponge selected, make sure you're correctly configured in the Tool Options palette at the top of the screen. Set the size of the tool so that you can paint on individual teeth easily. The tool shouldn't be larger than any individual tooth, but it shouldn't be so small that you need to do a lot of scrubbing. Also change the Mode to Desaturate--this will remove the yellow--and set the Flow (which controls the intensity of the tool) to 50 percent.

Now just start painting the sponge tool over each tooth. You should see the tooth turn a somewhat unappealing shade of gray. That's to be expected--we're currently desaturating the teeth--in other words, we're removing all the color from them. Mainly work the front-most teeth. The smaller teeth toward the rear will be in shadow, and you won't want to overwhiten them or the effect will seem artificial.

Now it's time to replace the gray by whitening the teeth. Go back to the same cubby where the Sponge lives and select the Dodge tool. In the Tool Options palette at the top of the screen, set the Range to Midtones and make sure the Exposure is pretty low--about 10 percent is just about right. If the exposure is too high, you'll know right away, since the teeth will look downright radioactive. Also set the Size to the same value you were using for the Sponge tool. Now just paint the Dodge tool over the teeth to change the gray to white. That's all there is to it.