Surface tension is the property of the free surface of a liquid that allows it to resist an external force. It is caused by cohesive forces between liquid molecules being greater than adhesive forces between liquid and gas molecules. Surface tension allows insects to walk on water and causes water droplets to be spherical. The angle of contact is the angle between the liquid-gas interface and solid surface when a liquid comes into contact with a solid. Surface tension can be measured using the capillary rise method, where the height risen in a capillary tube is directly proportional to the surface tension based on calculations involving tube radius, liquid density, and angle of contact.