Groups have a sense of identity but not expectations. A group of individuals who interact based on shared expectations and share some degree of identity. The outgroup is the network of relationships generated by a person's contacts with other individuals. In intergroup theory, groups are defined as all the individuals who are related to each other through descent from a single pair of ancestors. Groups may also be defined in terms of shared attributes or behaviors. For example, scientists use the term group when discussing the properties of molecules; these properties can be shared among similar molecules. Groups may also be defined in terms of function. For example, soldiers in a military unit are considered part of the same group because they perform tasks that help them achieve their goal of defeating an enemy. Groups may also be defined in terms of relationship. Parents and children tend to form a single group because they have a parent-child relationship with one another. Teachers and students form a single group because they have a teacher-student relationship with one another. Groups differ from classes in that classes are defined by a shared experience while groups can include people who have nothing in common except for their relationship to one another. Classes are defined by a set of standards that everyone in the class must meet in order for them to receive credit for the class. Groups do not have any such standards since they are only defined by the relationships between its members.