Why do airplanes have circular windows?
The sharp corners on square windows are places where stresses concentrate and multiply their effects, hence the term "stress risers." Cracks, buckles, and tears in the airplane skin tend to start at these points. The rounded corners in airplane windows are much less prone to this problem. Circular shapes are much stronger and resist deforming much better- ie like an egg! Also corners on external features such as windows would create much more drag on the air, meaning less fuel efficiency