Look at the corner of your copybook. Now look at the corner of the window, and the corner of the door.

Some corners are square and neat. Some are wide open or pinched shut. A square corner has a special name in maths. What do you think we call it?

A right angle is a square corner. Watch me make a checker for it: I fold a piece of paper in half, then in half again the other way. The folded corner is a perfect right angle, and I hold it against any corner to test it. You will make your own checker later on.

Watch me test four corners on the board. For each one, decide before I confirm: does the square corner fit exactly, is the corner too small, or is it too big?

Now we test corners together on the board. Sort each corner card into the right group: Right angle or Not a right angle.

Today we work through these together: the corner of a book, the corner of a party hat, the corner of a picture frame, and the corner of an open fan. For each one, decide if it is a right angle or not before you drop the card.

Time to be right-angle hunters. First, fold a scrap of paper in half, then in half again the other way — that folded corner is your own checker. Then, working at your own desk, use it to find:

• three right angles on objects on your desk,
• a right angle somewhere on the wall,
• a corner that is bigger than a right angle,
• a corner that is smaller than a right angle.