* Density is mass per unit volume. A denser object has more mass packed into the same space.

* Coin: A coin is made of metal (usually copper, nickel, or a combination), which is a very dense material. This means the coin has a lot of mass for its size.

* Styrofoam: Styrofoam is a lightweight plastic. Even though it may be the same size as the coin, it has much less mass. Therefore, it has a lower density.

Floating vs. Sinking:

* Objects less dense than water float. This is because the buoyant force (the upward force from the water) is greater than the force of gravity pulling the object down. The styrofoam displaces enough water to support its weight.

* Objects denser than water sink. The coin is denser than water, so the buoyant force isn't strong enough to overcome gravity. The coin sinks because its weight pulls it down more than the water pushes it up.