You likely encounter many different items throughout the day that can stain your clothes, particularly outside elements and food products. One thing you might never expect to stain your clothing is water, but hard water can do just that. What's actually staining your clothing are the minerals, including magnesium and calcium, that are present in hard water. If you notice this type of stain on your clothing, it can be removed with simple cleaning methods.
Place a large white towel on top your ironing board. Turn your iron to a setting that's safe to use on the clothing.
Take the clothing and turn it inside out. Lie it over the towel, making sure the water stain is on top of the towel facing downward.
Wet a cloth and lay it over the inside of the clothing directly over the water stain. Press it to moisten the clothing.
Iron the clothing on the same spot where the water stain is located on the other side. Continue to iron until you dry the clothing completely.
Move the towel so that the clothing is touching a new area. Re-wet the clothing using your wet cloth, and iron again until the clothing is dry. Repeat this process two or three times.
Turn your clothing right side in again, and rub any remaining water stain with the back side of a metal spoon. This should remove any remaining stains.