You can remove water rings from wooden furniture using a warm iron, mayonnaise or toothpaste if the furniture is less than 50 years old. Consult a professional furniture restorer for older furniture or if the ring turns black.
Remove stains as quickly as possible after discovering them for the best results. Empty the water reservoir from an iron before proceeding. Place a towel or napkin over the stain, and set the iron to low. Press the towel with the warm iron to heat the moisture and evaporate it from the wood. Keep the iron moving across the surface and check to see if there is progress after a few minutes.
If the heat does not work, the oil in mayonnaise is another option. Place a small amount on a rag and rub it into the stain. Allow the mayonnaise to sit on the stain for a minimum of an hour, reapplying more as necessary to keep the area moist. If you see results, you can leave the mayonnaise on the wood overnight.
Toothpaste provides a slightly abrasive substance to help remove the stain. Choose a variety that is not a gel, and place a small amount on a rag. Rub the toothpaste over the ring until it is no longer visible. Avoid rubbing the toothpaste on other parts of the wood as it wears away the finish. Clean the toothpaste from the furniture, and apply paste wax to restore the shine.