Grouping living things and using keys

Grouping by observable features

Living things are sorted by features anyone can check: number of legs, wings or no wings, backbone or no backbone, feathers, fur or scales. A good grouping question has a yes/no answer — “is it green?” is weaker than “does it have six legs?”

Vertebrates and invertebrates

Animals with a backbone split into five groups: fish, amphibians, reptiles, birds and mammals. Animals without a backbone include insects, spiders, worms and snails — and they are the large majority of animal species.

A branching key

A key is a chain of yes/no questions that ends at a name. Start at the first question, follow the branch your animal fits, and keep going. Each question must split the remaining animals into two — a question that everything answers the same way is a wasted step.