A key is a set of questions about the characteristics of living things. The answer to the first question gives you another question to answer and so on. As you answer more questions you narrow down your living thing until eventually the last question tells you what it is.
Watch: Classifying for two amphibian football teams
Choose something that interests you like your favourite films, pop groups, shoes or toys and see if you can make a classification key to sort them out.
Ask yourself a question to narrow down your choice to one and leave all the others in a group.
Ask yourself a second question to narrow down your choice to another one.
Keep on doing this until you only have one choice left.
Then to test that it has worked, give your key to your friend or member of your family to see if they can do it!