You finally get around to giving your refrigerator a much-needed cleaning. In the back of the bottom shelf you notice a container you don't recognize as one of your own. Suddenly you remember -- that's the leftover tuna casserole you brought home from your neighbor's potluck dinner six months ago.

You open it to find the tasty dish has turned into petri dish surprise.

If something looks like a science experiment in your refrigerator, it's easy to see it has gone bad. But even foods that still look fresh might not be good for you, and you can't always rely on an expiration date on a label.

Once you open a container, the food inside can go bad long before the date on the label. Check out these guidelines for knowing when it's safe to consume something -- and when it's time to toss it.

(If you don't see your favorite food on the list, or if you'd like more detailed information on how to store foods for optimum shelf life, check out this site or this one or this one.)

How Long Will It Keep?