Foods of various kinds are or were living things, or were made from them, and like any living things, they are subject to decay. Some things go from fresh and healthy to spoiled faster than we might think — especially now that the weather is getting warmer and, in some parts of the country at least, growing more humid as well.

When we say that foods “spoil,” we’re talking about several different things. Some develop mold or mildew. Both are kinds of fungus. Mildew is usually light in color and grows on the surface of food, while mold is typically black or green and present underneath the surface.

Some are affected by bacterial growth, becoming toxic and in some cases life-threatening if consumed. Some undergo enzymatic changes through oxidation, developing undesirable flavors and aromas. Some just get soggy or dried out, and thus unpleasant (or impossible) to use. Still others remain edible but lose their nutritional value and/or flavor.

A number of factors cause food to spoil, but the main culprits are moisture and exposure to air. Moisture provides a medium for the growth of bacteria and fungi, and fosters undesirable chemical reactions between components of a food. A lack of moisture can also be a problem, though, causing items to crack or crumble as they dry out.

Most bacteria and fungi are aerobic, meaning that they require oxygen to grow, and oxidizing enzymes cause food to turn brown. Extreme temperatures, high or low, and (for certain foods) light can contribute to food spoilage, too.

Most of the items listed here are raw materials. Bear in mind that cooked food spoils fast, too, sometimes more quickly than its individual ingredients. The Mayo Clinic advises that leftovers of any kind should not be kept for longer than three or four days in the refrigerator. If they’re not consumed within that period, they should be discarded or, if appropriate, frozen for future use. Uncooked foods, like sandwiches and salads, should be eaten as quickly as possible. To be on the safe side, you should know which foods should and should not be kept in the fridge.

Click here for 20 foods that spoil faster than you’d think.