You need to write at least 4 sentences to get a target.  If you write more than 6 sentences, you get a + plus.  If you write fewer than 4 sentences, you do not pass.  You get a -.  This is a “quad cluster”.  You are supposed to choose the term that is different from the other three.  Tell me how the one term is different and how the other three alike.  Don’t forget to grade yourself.

evaporation, condensation, clouds, water droplets