Introduction This section describes the principles and concepts related to the experiment. It is used to help the person reading your report to understand the information that serves as foundation for the experiment. It also includes a brief statement about the purpose of the lab. Example: Surface tension is the attractive force exerted upon the surface molecules of a liquid by the molecules beneath. Water has a high surface tension because there are strong attractive forces between water molecules. Adding surfactants, like soap, can reduce the surface tension of a liquid. The purpose of the experiment was to determine the effect of soap on the surface tension of water.