Whenever we think of windows we get the image of square windows into our brain first. But why are windows in plane are always rounded.

It is because to sustain pressure at higher altitudes. Anything with corners is tensile. A square has four corners that mean it has four weak points. In olden days Aeroplanes used to fly at greater heights to save time and at greater heights to survive people need pressure as the pressure decreases with altitudes. Square windows which are subjected to internal pressure of the cabin resulted in crash of the planes. Luckily technicians figured out the reason pretty soon. They replaced the tensile four cornered windows with round windows which do not have a weak point thereby eliminating crashing down of planes due to excess pressure.