Copper pots are considered to be the best kind available for cooking. To help them look brand new again, use salt and vinegar. Just sprinkle the surface with the salt, then pour a little bit of vinegar over it and rub with a hard sponge.
You will need a solution made of one part vinegar and two parts water. Mix the water and vinegar together in a spray bottle, then spray on the stain and cover with a damp cloth. Turn on the iron, set it to steam mode, and place it over the cloth for 30 seconds.