Here is a graph that displays an interaction:
Work Step by Step
The green line is the mean time to finish the exam for the open-versus closed-book exam, and the mean time to finish is identical for each type of exam. But if you look at just the open book exam, you see that the non-A students take much longer than the A students to finish. But the situation is reversed when you look at the closed-book means. With the closed-book exam, the non-A students take less time that the A students. This reversal in the data results is indicative of an interaction.