A penny won't actually rust in water as rusting related to iron, and a penny doesn't contain any. A penny will corrode in water over time, however, although this could actually take years to notice.
How do pennies corrode?
Pennies contain a certain percentage of copper, and this is vulnerable to corrosion in a process known as oxidation.
You've probably noticed that older pennies look pretty dark and don't shine as well. This is due to the process of oxidation, whereupon copper reacted with the oxygen in the air, forming copper oxide which coats the penny.
How to reverse the process?
The cool thing about pennies that have suffered from oxidation is that the process can be reversed. This is because the copper oxide forms a shell around the unaffected copper in the penny. All you need to do is remove the copper oxide, and your penny will look brand new. Here's how to do it:
- Simply fill a container with water, salt, and vinegar.
- Place the coin in the mixture and wait for a few minutes.
Here's a video showing you the process of cleaning pennies: