Those who love sitting by the window on airplanes are probably familiar with the small hole at the bottom of the window—a tiny mystery that plays an indispensable role in the engineering design of airplanes.

The small hole in airplane windows helps maintain safe air pressure inside the cabin and releases moisture, preventing the windows from fogging up.

As the airplane ascends, the air pressure decreases. The air pressure outside is significantly lower than inside the plane, creating harmful physical effects on the airplane window.

Airplane windows consist of three separate panes (outer, middle, and inner). The small hole in the middle pane, also known as a "bleed hole," helps balance the air pressure between the cabin and the small gap between the middle and outer panes. This means the outer pane takes on the full pressure, while the middle pane acts as a safety backup.

The inner pane doesn’t bear any pressure but helps protect the middle and outer panes from damage that could be caused by passengers. The "bleed hole" also releases moisture, preventing the windows from fogging up.