1. Your liver
2. Your skin
3. Your blood
4. Your fat
5. It’s a toss-up
5. It’s a toss-up.
It’s no surprise that your liver is an organ—in fact, at a little under 3 pounds of weight, it is on average the body’s largest “solid” organ. (The brain is a close second, and since there’s variation there are undoubtedly people whose brains weigh more than their livers—but that’s not the “toss-up” we’re talking about.)
The sneakier answer is the skin. Since it’s not all in one place, people don’t tend to think of it as an organ, but it is, because it consists of specific tissues and cell types and has vital functions. And the skin weighs much more than the liver, roughly 15% of a person’s total body weight—from around 15 pounds to more than 30.
As to blood, the average adult has from four to six quarts. Six quarts is 12 pounds, so it beats the liver, but blood is disqualified; it’s not an organ—it’s mostly water that carries lots of different kinds of cells and chemicals around the body.
Fat? For a long time, it was thought that fat cells were just places where fat is stored, but scientists know now that fat cells produce chemicals (hormones) that affect the operations of other cells in specific ways, including hormones that are supposed to let your brain know that you’ve got plenty of energy stored as fat and can stop eating. (“Supposed to” because sometimes that control mechanism doesn’t work as it should.)
Fat cells are a particular kind of tissue, and they produce hormones that affect other tissues, which makes them an organ, a specific type of organ called a gland. And depending on the amount of fat you have, it could weigh less than your skin, or more—there’s the toss-up.