Traveling by air can be bewildering. Why do flight attendants ask passengers to open my window shades during takeoff and landing? Why is the airplane shaking with turbulence, and is it dangerous? How do airport workers fit all of people’s suitcases in the luggage hold? And what on earth are those airport runway numbers painted on the tarmac?

Educational and hilarious YouTube channel CGP Grey answers that last question in a recently posted 17-minute video that people allergic to science should probably not attempt to watch.

First, the numbers on airport runways have a specific name — designation markings — and they are one of many airport marking aids and signs that are needed for pilots to take off, land, and taxi safely.

Airport runways are built in such a way that planes can take off and land into the wind most of the time. But because winds tend to shift, large airports often have more than one runway and they usually face different directions to avoid hairy crosswind maneuvers. In airports where the traffic is dense, there are also often parallel runways so all the aircraft departing and arriving at the airport can be accommodated in a timely manner. And the wind’s direction and the multitude of airport runways are the reason behind the designation markings: They help air traffic control to communicate safely with pilots as to what runway and which end of the chosen runway they need to take off from and land on.

But there’s more.

The airport runway numbers are not random digits. They are a rounded-up and shortened version of a compass number. For example, if a pilot is landing on a north-south runway and the wind is blowing from the north, they will be directed to the side of the runway with the designation marking 36 because it follows the compass heading of 360 degrees. However, if the wind is blowing from the south on that same runway, the pilot is to approach from other side, marked 18 because it has a compass heading of 180 degrees.

In the case of parallel runways, the designation markings are accompanied by letters: “L” for left, “R” for right, and “C” for central, allowing air traffic control to tell pilots exactly which runway they need to use.

And because the location of the magnetic north, on which all magnetic compasses are based, shifts over time, the airport runway numbers need to change accordingly, making for a substantial need for quick-drying white paint in airports. This is the case everywhere but Canada, where instead of using magnetic compass headings they use the unchanging heading from true north — the only country in the world that does it this way.