I've read about water heaters needing anodes, but I don't really understand why. The objective is that the anode should protect the tank by "attracting" corrosion, isn't that right? But aren't water heaters usually made of stainless steel anyway? Around here most water heaters don't have an anode, and if they do it's considered a special case, like if the water contains salt, which I guess attacks the tank even if it is stainless steel...? I also read that the anode is sometimes made of aluminium, which doesn't even corrode in the first place - how does that work?