The process of atomization often involves ejecting thin liquid sheets at high speeds from a nozzle that causes the sheet to flap violently and break up into fine droplets. The flapping of the liquid sheet has long been attributed to the sheet’s interaction with the surrounding gas phase. Our work shows that the flapping is in fact caused by the thinning of the liquid sheet as it spreads out from the nozzle exit. Our current work aims to predict accurately the final drop-size distribution so as to enable efficient designs of atomisers.