It's not called The Amazing Little Magic Sponge for nothing.
Environmentally friendly, removes tannin stains from teacups and teapots using only water, no chemicals and no hard rubbing.

Directions for use:  

Wet the sponge with water then gently squeeze.  
Rub stained area lightly then wipe dirt with a dry cloth or rinse with water.  
Rinse the sponge with water and re-use until it wears out.