A six-ounce can of tuna contains six ounces of tuna, right? Wrong. Under federal law, a so-called six-ounce can of tuna has to have only about 3.5 ounces (although the Tuna Foundation, which represents Bumble Bee, Chicken of the Sea and Starkist, has set a voluntary minimum of four ounces). The two- to three-ounce gap is essentially the difference between total weight and drained weight, which the label does not have to make clear. Sound fishy? There are a lot of food- and nutrient-related "facts" that people tend to take for granted but that just aren't true. Here are some others:

Assumption: Idaho potatoes are better than other potatoes.

Reality: Idaho potatoes are of the russet variety -- identical in quality to the russets grown in Colorado, Maine, North Dakota and several other states. (But they are grown in Idaho. "Idaho" is a registered trademark.)

Assumption: Smucker's fat-free butterscotch topping has fewer calories than Smucker's regular butterscotch topping.

Reality: The fat-free version has the same number of calories as the regular -- 130 per two-tablespoon serving. This also holds true for Jif peanut butter (190 calories per two tablespoons, "reduced-fat" or not) and Ore Ida Steak Fries (110 calories per three ounces, regardless of the "low-fat" label on some packages). Reduced-fat Yodels (the chocolate-and-cream version of a jellyroll) had more calories than regular ones -- 145 as opposed to 140 -- until the manufacturer, Drake's, discontinued them several months ago.

Assumption: Bananas have more potassium than any other fruit or vegetable.

Reality: A banana, with 451 milligrams of potassium, may have a popular reputation for potassium, but it is actually rather far down on the food list, surpassed by a baked potato (844 milligrams); a Florida avocado (742); a California avocado (549); five dried figs (666); a quarter-cup of raisins (563); a cup of cantaloupe pieces (494); a half-cup of Swiss chard (483); and a cup of orange juice (474). Guidelines recommend consuming at least 2,000 milligrams of potassium daily, but studies suggest that 4,000 to 5,000 milligrams is better for keeping down blood pressure.

Assumption: If you eat dolphinfish, you're eating Flipper.

Reality: Dolphinfish is another name for mahi-mahi ("strong-strong" in Hawaiian), a type of fish that has nothing to do with mammalian dolphins.

Assumption: A tough steak contains more fiber than a tender one.

Reality: Steak doesn't contain any fiber at all. Fiber is only in plant foods: grains, fruits and vegetables.

Assumption: If a recipe says to preheat the oven and you don't, the food will not turn out right.

Reality: All that preheating is unnecessarily adding to your gas or electric bill. A number of studies dating back to the 1930s show that all manner of dishes, from angel food cake to biscuits to beef patties to cream puffs, cookies, pie shells and souffles, come out just as well from non-preheated ovens as preheated ones. They may need a few extra minutes, but not the 10 minutes or longer that most cooks use for preheating.

Assumption: Oranges whose rinds have some green on them are not ripe yet.

Reality: Green oranges (and grapefruits) can be just as ripe as orange ones. Color differences are due not to ripeness, explains the Florida Department of Citrus, but to differences in nighttime temperatures before the fruits are picked. A cool temperature at night gives orange skins a deeper orange hue. If the nights are warmer, some green will remain. Instead of checking for color, check for rinds that are smooth and free of soft spots. And make sure the fruit feels heavy for its size. The heavier, the juicier.

Assumption: A heavy coffee drinker who switches from regular to decaf will not have to keep running to the bathroom.

Reality: While caffeine is a diuretic, decaffeinated coffees (and teas) may contain bladder irritants that could still keep you "going," some researchers speculate. Moreover, coffee is mostly water. The more water you drink, the more you will need to get rid of.

Assumption: Wisconsin produces more milk than any other state.

Reality: California is the top milk producer, reports the California Dairy Board. But Wisconsin still produces the most cheese.

Assumption: A chocolate-caramel bar is worse for your teeth than a cracker.

Reality: Pieces of cracker stick to your teeth longer than bits of caramel candy, research indicates, providing more fodder for the bacteria that release cavity-causing acids. The simple sugars (carbohydrates) in caramels appear to be washed out of the mouth relatively easily by saliva as well as by the movement of the tongue. Starchy (complex-carbohydrate) foods like crackers crumble and can cling to the teeth for hours. Brush after eating, whether or not the food was sweet. And floss at least once a day.

Assumption: It takes a lot of green vegetables to equal one serving.

Reality: No, it doesn't. A half-cup (one serving) of cooked green vegetables comes to four Brussels sprouts, five broccoli florets or six forkfuls of spinach. Health professionals recommend three servings of vegetables a day, one of them green.

Assumption: A pre-mixed salad in a sealed bag is pre-washed and ready to serve.

Reality: A pre-mixed salad in a sealed bag is only ready to serve if the bag contains the words "washed" and "ready to eat." Otherwise, wash it to rinse off harmful bacteria like salmonella. The same is true for pre-cut and peeled baby carrots and other ready-looking vegetables.

Assumption: Red wine protects against heart disease better than other types of alcohol.

Reality: Beer is health food, too. The ingredient in alcoholic beverages that explains just about all of their protective effect is the alcohol itself. There is no clear evidence that substances particular to red wine confer extra benefit, says Harvard researcher Eric Rimm.

Assumption: Green tea is caffeine-free.

Reality: No, it isn't. A cup brewed from a Lipton bag has 30 to 50 milligrams of caffeine -- about the same amount as in a bag of Lipton's black tea.

Assumption: Eating a big lunch, especially a starchy one, makes you sleepy in the afternoon.

Reality: A wave of drowsiness is likely to hit during the midafternoon regardless of how much -- or what -- you eat. It's the body's natural circadian rhythms at work.

Assumption: The larger jar, box or can always costs less per pound than the smaller one.

Reality: Not always. Tuna fish, peanut butter, ketchup, canned coffee and other foods sometimes cost more per pound in their larger containers. Check unit prices (price per pound) on the edges of store shelves.

Assumption: You wash your hands thoroughly enough before preparing food.

Reality: You should be able to sing "Happy Birthday" twice while washing your hands. It takes a full 20 seconds to do it properly, says the International Food Safety Council.

Assumption: Americans are the fattest people in the world.

Reality: The Samoans are the fattest, according to the World Health Organization. But we are among the fattest.