If you’ve ever had a window seat on a flight, you have probably noticed a tiny hole at the bottom of the window.

Most airplane windows are made up of three panels of acrylic, so when you look through the plane window, you are actually peering through three different panes. The exterior window works for keeping the elements out and maintaining cabin pressure. If something happens to the exterior pane, the second pane acts as a fail-safe option.

The tiny hole is in the middle one and is called the “bleed hole”. Its primary purpose is to balance air pressure. There’s a small gap between the middle and the outer panes. The “bleed hole” allows pressure to balance between the passenger cabin and the air gap.

The “bleed hole” plays an important role in keeping us safe, it also helps keep the window panes from fogging up or frosting over – the result of the temperature difference between the inside and the outside of the cabin – allowing us to stare out into the clouds.

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