means seeing relationships between and among things in our environment. Relationships might involve interactions, dependencies, and cause and effect events. In the sciences, information about relationships can be descriptive (as it is in much of ecology) or experimental (as it generally is in physical science). Descriptive relationships are derived in situations where direct experimentation is not possible. Many of the ideas in astronomy are based on description. We cannot do direct experiments on galaxies or black holes, yet we know a great deal about them. Determining relationships experimentally frequently involves controlling and manipulating variables in such a way that the scientist can determine which of the factors affects the observed event the most. The ability to separate variables and systematically test each one while holding others constant to determine which are relevant and which have no effect is very powerful. By manipulating variables the scientist is able to test hypotheses.