Key concepts
Body composition
Muscle and fat

From National Science Education Standards: Characteristics of organisms

Your body has a lot of different kinds of materials in it. There are, of course, bone, blood, fat and muscle—just to name a few.

But all of these parts are hidden away under our skin, so how can we learn more about some of their qualities? Animals have a lot of the same insides as we do, so we can learn some interesting things about our bodies by studying something as basic as meat you can buy at the store. One easy and fun test to do is examining whether substances float in water—which tells us how buoyant they are. More about buoyancy in a moment.

Some components of the body are denser than others. Try this activity to discover which substance is denser than water: muscle or fat. If a substance is less dense than water, it will be buoyant, meaning it will be more likely to float.

Density is determined by an object's mass and the amount of space it takes up (its volume). This is why heavy metal ships can float on the water—they're big enough to have an overall density that's less than that of water, but if you crushed them down into a ball and got rid of all of the empty space, the metal ball would sink, well, like a rock!

Within most of the human—and animal—body, whether muscle, fat, blood or bone, is a lot of water. This means our bodies really are close to the density of water. But this activity can also help explain why some animals—and people—are more buoyant than others.

•    Cooked piece of meat that has both lean meat and fat on it (such as a pork chop or steak)
•    Knife to cut the meat
•    Piece of bread
•    Large clear glass cup or bowl
•    Water

•    Fill the clear glass cup or bowl with water.
•    Tear off two big pieces of bread that are about the same size.
•    Set one on the counter as is, and smash the other into a dense ball.
•    Carefully cut a piece of fat off of the meat.
•    Carefully cut a piece of lean meat that is about the same size as the fat.
•    Do you think any of these four items will float in the water? Why or why not?


•    Place the cut piece of animal fat in the water. What happens?
•    Now put the cut piece of lean meat in the water. What happens to it?
•    Now place the un-smashed piece of bread in the water. What happens?
•    Remove the bread, and now place the densely smashed ball of bread in the water. What happens to it?

Read on for observations, results and more resources.

Observations and results
Why did two different materials from the meat that were the same size (thus had about the same volume) do different things in the water? How could two pieces of bread that started out the same (thus had about the same mass) do different things in the water?

These two different results happened because the objects had different densities (which is determined by a combination of volume and mass). If an object has a greater density than water, it sinks. If it is less dense than water, it floats. Which type of body material—muscle or fat—had greater density than water and which had a lower density?

So, as it turns out, athletes with very little body fat might have to work harder to stay afloat in the water. To be healthy, our bodies need a balance of fat and muscle.

Some animals that live in the ocean have a thick layer of fat called blubber. Blubber helps to keep these animals warm, but how else might it affect a whale or manatee in the water?

Have you ever been swimming in the ocean or a salty sea? If you have, you might have noticed that it's easier to stay afloat in the saltwater than it is in a lake, pond or swimming pool. Why is that? Salt dissolved in the water makes it more dense. So even if you haven't changed your body composition, you are less dense relative to the salty water, which helps you float on the surface. You can demonstrate this effect at home by filling two glasses with warm water and dissolving about six tablespoons of salt into one of them. Gently place an egg in each glass, and see what happens.

Share your muscle versus fat observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Pour out water and throw away bits of bread, meat and fat (or compost them if you can).

More to explore
"Going for the Gaunt: How Low Can an Athlete's Body Fat Go?" from Scientific American
"Do Giraffes Float?" from Scientific American
"Floating Eggs In Salt Water" activity from Reeko's Mad Scientist Lab
"Your Muscles" overview from KidsHealth
Will It Float or Sink? by Melissa Stewart Page, ages 4–8
Head to Toe Science: Over 40 eye-popping, spine-tingling, heart-pounding activities that teach kids about the human body by Jim Wiese, ages 9–12

Up next…
Color Changing Dots

What you'll need
•    Five to 10 different colors of construction paper
•    Hole puncher
•    Bowl
•    Colored fabric or different color surface (optional)
•    Timer (optional)