An "if, then" statement is an example of a scientific hypothesis; for instance, "If I brush my teeth daily, I won't develop cavities." The null hypothesis, a statement that there is no difference between two variables, is another hypothesis type. For example, "All trees have the same number of branches."
When a scientist disproves a null hypothesis, it means there is a relationship between two variables. Scientists also use "if, then" statements to make educated guesses about the relationships between pairs of variables. A good hypothesis has an independent variable and a dependent variable. Scientists often design experiments based on hypotheses to examine the effect any change the independent variable has on the dependent variable.