Here's why:

* Passive Diffusion: This is the movement of molecules from an area of high concentration to an area of low concentration. It doesn't require energy.

* Oxygen in the body: The concentration of oxygen is higher in the blood and the surrounding fluids than inside cells. This concentration gradient drives the movement of oxygen into the cell.

Important Note: While oxygen enters cells by passive diffusion, the overall process of gas exchange in the lungs is more complex and involves active processes like breathing and the action of red blood cells.