WHY? is our attempt to answer all the questions every little kid asksHave a question? Send it to why@mentalfloss.com.

I bet you know that wood floats but a rock sinks. But why? Let’s look deep inside each object at its molecules (MOLL-uh-kyools). Everything is made of molecules. Molecules are very, very tiny. You can only see them with a special microscope. In some objects, like a rock, molecules are squished tightly together. In others, like wood, the molecules are more spread out. How closely molecules are crowded together in a space is called density. Density is a big part of why some things float and others don’t.

Objects like coins, rocks, and marbles are more dense than water. They will sink. Objects like apples, wood, and sponges are less dense than water. They will float. Many hollow things like empty bottles, balls, and balloons will also float. That’s because air is less dense than water. This is one reason huge ships can float, even though they’re very heavy. Inside a ship, there’s a lot of hollow space filled only with air. But that’s not all: shape matters, too!

The outside of an object is called its surface. When more of the surface is touching the water, the object is more buoyant (BOY-ant), which means it floats better. When an object floats, it pushes water out of the way. That’s called displacement. But guess what? Water pushes back! So the more surface area an object has, the more water pushes back against it, helping it float. That's another reason why even big ships don't sink if they have the right shape. 

You can test this out yourself. Try making a boat with a flat bottom using tinfoil. Does it float? What happens if you start putting pennies or pebbles inside your boat? What happens if you crumple your tinfoil boat into a ball? Using what you’ve learned about density and buoyancy, why do you think the empty boat floats best?