Distinguish complete metamorphosis (four very different stages, like a butterfly) from incomplete metamorphosis (three stages, with a nymph that resembles a small adult, like a grasshopper).
Metamorphosis means a big change in body form during a life cycle. Some insects go through complete metamorphosis, which has four very different-looking stages: egg, larva, pupa, and adult. A butterfly is the classic example; its caterpillar (larva) looks nothing like the winged adult, and the pupa (chrysalis) is a resting stage where its body dramatically rebuilds itself.
Other insects go through incomplete metamorphosis, which has only three stages: egg, nymph, and adult. A grasshopper nymph hatches already looking like a tiny, wingless version of the adult. It molts, meaning it sheds its outer skin, several times as it grows bigger and grows wings, without ever having a resting pupa stage.
Mastery looks like: They can correctly sort a new insect example into complete or incomplete metamorphosis and explain the key difference between the two.
Common stumbles: Assuming all insects go through a caterpillar-to-butterfly style change, or confusing the terms nymph and larva.