Why windows are round in airplane ?

why airplane window are round rather than square ?

At the time of 1950s ,the first commercial jetliner,the de Havilland comet started. but some time later   3 comet plane was crashed  simultaneously ,investigation report says that it is due to fuselages tore     apart during flight .                                                                                                                                     

In simple word due to change in air pressure  between ground and sky, the fuselage structure also   change because fuselage is also  pressurized . Airlines fly aircraft at higher altitude to save fuel.This

cause stress on fuselage ,in this square window cause more obstruction in stress  compare to round window .                                                                                                                                               

If window is square there is more chance to wreck plane structure so round shape is batter choice   instead of square .                                                                                                                                  

 This figure help to batter understand that square shape creating more obstruction in stress  than round shape.                                                                                                                                                       

sharp corner cause more obstetrical   means more stress difference leads to damage of fuselage .