The word that joins the class

Learning outcome

Given three words that already share a class, choose the option that joins that class.

Concepts and reasons

This is classification read forwards. The stem shows the class. Iron, copper and zinc are metals, so the word that joins them must also be a metal. Silver joins the class. Oxygen does not.

Square, triangle and pentagon are plane figures. Hexagon is also a plane figure, so it joins them. Cube is a solid, so it does not.

Worked examples

Example 1. Iron, copper, zinc. Which joins them: silver or oxygen? Silver is a metal. Oxygen is not. Silver joins the class.

Example 2. Square, triangle, pentagon. Which joins them: hexagon or cube? Hexagon is a plane figure with straight sides. Cube is a solid. Hexagon joins the class.

Common traps

A familiar neighbour is not enough. Cube sits near the other shapes in a textbook chapter, but the class in the stem is plane figures, not every shape.

Practice questions

1. Carrot, radish, beetroot. Which joins them: turnip or potato?
2. Name the class before you look for the joining word.

Answers and explanations

1. Carrot, radish and beetroot are roots. Turnip is a root, so turnip joins them. Potato is a stem tuber, so it does not.
2. The class has to be fixed first. Otherwise a related word from a wider class looks acceptable.

Analogy

Iron, copper, zinc, and the joining word silver.

The stem is three metals. Silver is a metal, so it joins them. Oxygen is a non-metal and stays out.

Quick reference

• Read the three given words as a finished class.
• The answer must be inside that class, not merely nearby.
• Metals: iron, copper, zinc, silver.
• Plane figures: square, triangle, pentagon, hexagon. A cube is a solid.