Final year students – a lot of you are worried about “interaction diagrams” in 2-way ANOVA and what they mean. This page looks very useful and should answer all your questions with some very simple examples. JUST READ THE FIRST SECTION – “Introduction”. Stop when you get to “II. Notation” or you will get a headache. You have been warned.
I think the really interesting examples are those where neither factor is significant, but the interaction is, and you get an X-shaped diagram. First and second year could usefully look at this, too.