Look at the two clocks on the board. On the first clock, the minute hand points to the 12 and the hour hand points to the 3, making a neat square corner. On the second clock, the hour hand moves down to the 6, so the two hands make one flat, straight line.

Four angles on the board, each drawn on a protractor so you can see how far it opens. Compare each one to a right angle, the square corner.

Watch which is smaller than the square corner, which fits it exactly, which is bigger, and which opens all the way to a flat line.

Look carefully at the last one. A straight angle is not a wide corner, it is a flat line, a half turn. How many right angles would fit inside it?

Now we make each kind of angle ourselves on the board. A pupil drags the ray to open the angle, and the class decides which group it belongs to before we read what the protractor says.

Key point The test is always the same: hold the paper right-angle checker against the angle. Is the square corner too big to fit inside it, does it fit exactly, does it slot in with room to spare while the arms still bend, or do the arms lie flat in one straight line?

Now we make each kind of angle ourselves and sort it. Open the angle on the board, check it against the corner of your copybook, then drag the matching card into its group.

First open it a little to make an acute angle. Then open it to an exact right angle. Next open it wider for an obtuse angle. Last, open it all the way so it makes a straight angle, one flat line.