With mouth-watering thoughts of a tasty sandwich on your brain, you unwrap a loaf of bread and see a patch of green mold. Do you cut off the gross part or chuck the entire loaf?

Maybe you hear the voice of your mom or dad in your head. ("Don't be a baby! Just eat around it!") Or you feel the guilt of food waste. (By some estimates, about one-third of all the bread made in the U.S. is never eaten.)

But science seems to overrule guilt and parental pressure, at least in this instance.

"We don't recommend cutting mold off of bread, because it's a soft food," Marianne Gravely, a senior technical information specialist for the U.S. Department of Agriculture, tells NPR. "With soft food, it's very easy for the roots [of the mold], or the tentacles, or whatever creepy word you want to use, to penetrate" deeper into the food.

Yes, better throw that loaf away.

Molds and mushrooms

Molds are a type of fungi, and come from the same family as mushrooms. In fact, if you look at molds under a microscope, they look like very skinny mushrooms, according to the USDA. They have a stalk with spores on top that form the colorful blue-green burst that you see on food.

What you don't see are root threads underneath that invade the foods it lives on. This network of roots is microscopic and impossible to see. So what looks like "clean" bread may actually be teeming with this tangle of entwined mold threads. In addition, moldy foods may also have invisible bacteria growing along with the mold, says the USDA.

But what if your bread is sliced, offering each piece a bit of separation from the next? Can you grab a slice a little farther down the loaf or is the entire thing shot?

There's a chance that the mold hasn't spread from slice to slice. But Gravely points out that mold can make the jump. In fact, legions of mold spores could be dancing around the bread in your bag — like dandelion seeds blowing across your lawn.

What about moldy cheese and other foods?

Most hard cheeses can be saved from mold if you cut carefully. (Photo: IVL/Shutterstock)

You might decide to toss half a loaf of moldy bread, but can you make the same call when you spot mold on a block of cheese or even on some fruits and vegetables? Here's what the USDA has to say about safety. (Food waste is a whole different issue.)

• If something is moldy, don't sniff it. That can lead to respiratory issues.
• When throwing away moldy food, put it in a small paper bag or wrap it in plastic. Discard it in a covered trash can where children and animals can't reach it.
• Clean the refrigerator or pantry at the spot where the moldy food was stored.
• Check items stored near the moldy food. Mold spreads quickly, particularly in fruits and vegetables.

Most foods, including lunchmeat, soft or sliced cheeses and soft fruits should be thrown away if mold appears. But there are a few foods that can be salvaged, according to the USDA.

Some cheeses like Gorgonzola and Roquefort are created with mold that's safe to eat. But if you have other cheese in your fridge that suddenly develops an unusual blue-green tinge, you can still save it. If you find mold in hard cheese, cut it off at least one inch around and below the mold spot and then re-cover the cheese in fresh wrap. You can do the same for firm fruits and vegetables such as carrots and cabbages. Here's a complete list of moldy foods and when to use or discard them.

For example, always throw away moldy peanut butter, jellies, yogurt, casseroles and pasta.

Can mold make you sick?

If you gave in to that sandwich craving or didn't notice the mold until you took a bite, there's a chance you'll be fine. Many types of mold are harmless to people. But some molds can trigger allergic reactions and respiratory issues. In some cases, a few molds produce mycotoxins, poisons that can make people and animals very sick.

Because you don't know which one of those molds is lingering on that slice of bread, it might be a better idea to forget the sandwich and just make an omelet instead.

This video from Science Insider takes a deeper look at the whole moldy bread question.