There's a number of travel conundrums that might run through your head while you're on a plane - and why the windows have little holes in them is one of them.
Now, thanks to a report in the Telegraph, we're all enlightened.
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Having little holes in the window seems like a terrible idea but, of course, there's a good reason they're there.
So here's the science bit: at high altitude the air is less pressurised and contains less oxygen; to make sure those on board don't lose consciousness, the cabin air on the inside of the plane is pressurised.
This puts great pressure on the windows. Plane windows are triple-glazed, with the two outer panes taking the most pressure. That's where the 'bleed hole' comes in - it relieves some of the pressure.
Speaking to Telegraph Travel, Justin Dubon, of Airbus, said: "The middle pane is the one with the small hole in it.
"The purpose of that hole is to allow pressure to equalise between the passenger cabin and the air gap between the panes."
Thanks to the Telegraph's Travel Truths series, you can also find out what happens when you flush a plane loo (c'mon, we've all wondered that), and why cabin lights are dimmed during landing and take-off.