Key point A square corner is a right angle. If an angle opens less than the corner it is acute. If it opens more than the corner it is obtuse.

Four angles on the board, each drawn on a protractor. Take them one at a time and decide, before you read the label, whether it is smaller or bigger than a square corner.

The last two are the ones that matter. They sit either side of the corner by only a few degrees, so guessing by eye is not enough.

Pupils take turns at the board while the class watches and agrees aloud. We take the angles one round at a time. First we sort a clearly narrow angle and a clearly wide one. Then we look at an exact square corner. Last we check one that is only just bigger than a right angle. For each card, decide whether the angle is smaller than, the same as, or bigger than a square corner.

Key point A right angle is exactly 90 degrees. Under 90 is acute. Over 90 is obtuse. That one number settles every card, even the ones that look almost the same.

Four rounds, getting harder: clearly acute and clearly obtuse, then angles within a couple of degrees of a right angle, then a set with exact right angles in it, then tilted angles. A tilted angle opens the same amount no matter which way it points.