If you drop an unpeeled orange in fresh water, it floats. If you drop the same orange in a bowl of salt water, it also floats. The unpeeled orange floats because it is less dense than both fresh water and salt water.
If you peel the orange before dropping it in the water, it sinks. Although the unpeeled orange weighs more than the peeled orange, its peel traps pockets of air around the flesh of the fruit. These air pockets make the unpeeled orange less dense than water, which gives it buoyancy and causes it to float in water. When the orange is peeled, the trapped air is released. The peeled orange sinks because it is denser than water. Students often believe the misconception that light objects float and heavy objects sink. However, if the object's density is lighter than the liquid's density, the object will float in the liquid, regardless of its weight. That is why a cannonball will float in liquid mercury.
If you add salt to the water in the bowl, the peeled orange will start to float. Although the weight of the peeled orange does not change, it floats because it is less dense than salt water. Because salt water is denser than fresh water, objects that do not float in fresh water sometimes float in salt water. Objects that do float in fresh water float higher in salt water. An example of this is how easily people float in the Dead Sea, which has an extremely high concentration of mineral salts. In fact, people float so high in the Dead Sea they can read newspapers while relaxing there.
Just like the orange's peel affected its density and buoyancy, a person's body composition affects his natural ability to float in water. People who have greater muscle density or a lower ratio of fat-to-muscle fiber tend to sink in water. However, because of lung capacity, our human bodies tend to float more than they sink. A person with a large lung capacity floats easier than someone with less capacity because lungs fill with air and give the body buoyancy.