It's not just because they're cute.

Airplane windows are mostly considered Instagram-worthy frames for travel photos taken while inside a plane, but there's actually a science as to why they're circular in shape. The shape of airplane windows is even considered one of the most important innovations in aviation.

Apparently, it's one of the reasons why airplane passengers are kept alive while flying mid-air.

A new video made by Real Engineering explains in detail how round airplane windows ensure safety mid-flight.

When jet engines and pressurized cabins were introduced, commercial travel became well known. It allowed and required airplanes to fly at higher altitudes so as to elude turbulence.

However, the pressure difference between the plane's cabin and the air outside it also became greater. This pressure difference makes the plane expand very slightly, but it also creates much stress. Here's where the circular windows come in. They prevent stress levels from rising further and causing the plane to crack. Those Instagram-worthy windows enable the plane to withstand the internal pressure.

"Stress will flow smoothly through a material, but if an obstacle is placed in its path, like a window, it needs to change direction. This causes the pressure to build in certain locations. This is called stress concentration," the video explains.

According to the Smithsonian, there was a time when airplane windows were squares. That's the shape of the windows on the de Havilland Comet, the world's first commercial jet. When the Comet first flew in May 2, 1952, it flew at an unexpected 35,000 feet speeding at 460 miles per hour.

However, by 1953, three planes had tragically crashed, killing 43 passengers in their wake. The planes just "disintegrated" mid-flight, and all Comets were forced to stop operations until the cause was determined.

Later on, engineers figured out that the square windows couldn't stand the pressure building on its corners as the planes flew, leading to the body of the planes expanding and cracking.

As written on the Smithsonian:

"At high altitude, after many pressurization cycles, the Comets' fuselages simply lost their ability to contain high air pressure, and the planes exploded with bomblike force."

Thankfully, engineers eventually discovered that round windows are better at handling pressure.

Today, all airplanes need to have round windows, which would first need to undergo arduous FAA testing before flight, according to Mashable.

Unfortunately for the Comet, the reinvention of the round plane windows was too late. Comet 1 didn't get to fly again, as well as Comets 2 and 3, according to USA Today Travel.

Check out the full explanation as to why plane windows are made round in the video below.