In this paper, a qualitative explanation of the causes of surface tension forces at a liquid–gas interface acting parallel to an interface is presented. None of the common explanations of surface tension given in preliminary physics courses consider the reasons behind the attractive forces between nearest neighbor molecules acting parallel to a surface. The earlier proposed explanations are based on the assumption that there is a concentration gradient at the liquid–gas interface. The analysis is based on considering the anisotropic pressure tensor in the interface layer. Unfortunately, these explanations are difficult to understand in preliminary physics courses. Additionally, they do not pay enough attention to the physical reasons for the formation of a special surface layer with the distance between the nearest neighbors in the direction parallel to the surface greater than in the direction normal to it. In this paper, a simple qualitative explanation of surface tension forces at a liquid–gas interface is provided. The consideration includes the reasons for the formation of a special liquid–gas interface and the reasons for the appearance of tangent surface tension forces. The instability of the liquid–gas interface and the associated fluctuations are also discussed.