Here's the breakdown:

* Density: Density is a measure of how much mass is packed into a given volume.

* Buoyancy: The upward force that a fluid (like air) exerts on an object immersed in it.

* Floating: An object floats if the buoyant force acting on it is equal to or greater than the object's weight.

Why most things sink in air:

* Air is very low density: Compared to most solids and liquids, air has a very low density.

* Gravity: Gravity pulls objects downward, and the downward force (weight) of most substances is greater than the upward buoyant force from air.

Examples:

* A rock: A rock is much denser than air. Its weight pulls it down, and the air doesn't provide enough buoyancy to counter that weight.

* A feather: While a feather is lighter than a rock, it still has a higher density than air. The air provides some upward force, but not enough to overcome the feather's weight.

Exceptions:

* Hot air balloons: The air inside the balloon is heated, which makes it less dense than the surrounding air. This difference in density creates an upward buoyant force that lifts the balloon.

* Helium balloons: Helium is a gas that is less dense than air. This density difference allows the balloon to float.

In short, most substances sink in air because they are denser than air and gravity pulls them down more strongly than the air can push them up.