We've long espoused the benefits of booking the window seat, but spend enough time there, looking out of that more-or-less 9-by-12.5 inch quasi-rectangle, and you'll begin to notice some things. How small the world looks from above. How the plane's wings glide through the clouds.

Another thing? A tiny hole.

The hole isn't just there for aesthetic disruption. Instead, the hole — tiny as it may be — helps keep passengers safe.

It all comes down to pressure: the higher the altitude, the lower the oxygen, which can leave people feeling ill and short of breath. This will be a familiar experience, for example, for anyone who has recently been to Taos, New Mexico; or anyone who has recently climbed a mountain.

At a cruising altitude of 35,000 feet, the air pressure is so low that anyone would pass out if they were exposed to it, so a plane's cabin must be pressurized to be much greater than the outside air, reports The Telegraph. Good for passengers, yes, but not so great for the plane, which needs a way to release some of the strain this puts on the aircraft.

Enter the tiny "bleed" or "breather" holes, which do just that.

Look closely at your window, and you'll see that it's not just made up of one durable pane, but three. According to Mark Vanhoenacker, a British Airways pilot who writes for Slate, the innermost pane is mostly to protect the second and third panes, which are "designed to contain this difference in pressure between the cabin and the sky." The bleed hole, then, allows pressure to be balanced out between the cabin and gap between panes.

Another function of the airplane window hole? To release moisture and minimize the frost or condensation blocking your view. That's good news for your Instagram.