9. The Little Hole In The Side Of A Pen
At a first glance, it might strike you as just a design but it is there for a reason and meant to serve a good purpose. Without the hole, a pen will be completely airtight and as you write and use the ink, the pressure will fall preventing the ink from flowing to the tip and eventually the pen will cease to write although there is still ink in it.
On highlands or in planes where the pressure is lower, a pen with no hole can explode soiling your attire with ink. The hole equalizes the pressure in the pen to allow it to write. So the next time you see it, you should know it is not just there for fun.