I have this aluminum 2-quart saucepan that has black stains on the bottom of the inside of the pan. They got there because I boiled water in the pan.

Does anyone know the science behind why this happens?
More importantly, does anyone know how to remove those stains?
Lastly, how can I prevent this from happening again? I like to use this saucepan as the bottom of a doubleboiler and it doubles as my one and only pan for candy-making. (It's my only non-Teflon pan).

Thanks in advance for all the help,
lori