If you haven’t guessed already, the holes have something to do with regulating cabin pressure. Most cabin windows consist of outer, middle and inner panes – all of which are made of a superstrong synthetic resin. It’s the middle pane that typically has these holes in them. While the inner pane is merely a failsafe (just in case the outer pane fractures), the other two panes are structural.
Cruising at 10,600 meters (35,000 feet), the pressure is around 1.5 kilograms (3.3 pounds) per square inch. This is too low for the human body to stay conscious, so the pressure is artificially maintained at around 3.5 kilograms (8 pounds) per square inch. Of course, if you increase pressure inside, the structure has to be strong to hold the difference between the external pressure and internal pressure.
According to Gonzalez, the little hole is there to act as “as a bleed valve, allowing pressure between the air in the passenger cabin and the air between the outer and middle panes to equilibrate.”
In short, in means the strongest outer pane is left to do the proper work, while the middle pane is available in case of emergency. So next you’re on your EasyJet flight to Mallorca and you see a wee little hole in your window, don’t panic, take a deep breath and hit your family with some epic science chat.