What do airplane windows and ballpoint pens have in common?

Airplane Window by Heute

They both have a tiny hole on their side. Why? Well, pressure, of course.

If you’ve ever taken a window seat on an airplane you might have noticed that there is a tiny hole at the bottom of the window glass. Even though it is quite a small hole one can’t help but wonder what its purpose is. After all, if there is one thing you don’t want to see on a plane is a hole.

That hole, however, serves a very important purpose. As you might be aware, airplanes are pressurized to create a comfortable and safe environment for passengers and crew at high altitudes where air pressure and oxygen levels are too low for humans to survive. Otherwise, we would either freeze or pass out from the lack of oxygen. None of these options are ideal for passengers, let alone for the pilots and the rest of the crew.

However, the difference in air pressure between the outside and the inside of the aircraft creates tension between the glass panes of the aircraft’s windows and if left unchecked could cause the window to crack or even shatter. I cannot stress this enough, holes on an airplane are a big no-no.

The easiest way to fix this would be to not have windows at all. Unfortunately, that would make for a very unpleasant experience for the passengers and would also create a safety issue making it harder for passengers, crew, and potential rescuers to navigate their way in and out of an aircraft in case of a crash. This is where those tiny holes come in.

Called ‘bleed holes’ or ‘breather holes’ they are typically located in the middle layer of the three-layer aircraft window design, and their  purpose is to regulate pressure between the different glass panes and prevent fogging.

This hole allows the air pressure to equalize between the panes, stopping the outer glass from bearing the full brunt of the pressure changes as the airplane ascents and descends during flight, preventing it from shattering. By allowing air to circulate between the panes it also keeps moisture from accumulating and compromising visibility.

In case of an emergency landing, a regular window would most likely shatter due to the rapid changes in pressure as the plane quickly descends. This tiny hole prevents that from happening by keeping the window’s structural integrity even in the most extreme scenarios.

BIC Cristal Pen by Wikimedia Commons

Now that you have some grasp of why these holes are so important for an aircraft, you might be wondering why a ballpoint pen would also need one. After all, unless you are using it during an exam, it’s probably not going to find itself in many high pressure situations.

Although a regular ballpoint pen will probably not ever fly to a high altitude, there is still a difference in air pressure between the inside of the pen and the outside. The principle is the same as that of an airplane window. The small hole on its side is there to equalize the air pressure between the inside and the outside of the pen and to prevent the ink from leakage.

Ballpoint pens work by applying pressure to the tip of the pen which makes the ball rotate and the ink to come out through capillary action – the movement of a liquid within narrow spaces, such as a tube or a straw.

To put it simple, a liquid in a narrow tube like the one that a pen uses to store its ink will always be drawn upwards unless there’s a force preventing it.

This way, by keeping the pressure equalized, the hole on the side of the pen keeps the ink from leaking while the pen is not being used.

Bad habits die hard

It’s not just the pen itself that has a hole in it. The pen cap also has a hole on its top. This however has nothing to do with pressure but it does have to do with air, particularly with breathing.

Even if you don’t do it yourself you probably have at least one friend who likes to nibble on pen caps. You know. The one you never asked to borrow a pen from because they always looked gross and chewed on.

Because of your friend and of millions of other people like them, pen caps have a hole at the top so that if they ever happen to swallow and choke on it, the cap will not stop them from breathing, by allowing the air to pass through its top.

It’s literally a safety issue that prevents people with bad chewing habits from dying in a horrible manner.