A grid is filled with coloured pieces except for one gap the shape of a 4-square piece. The student picks which of four pieces fills it exactly: a straight line, a 2 by 2 square, a T, an L or a zigzag. Every choice is 4 squares, so counting squares isn't enough.
The right piece is usually drawn turned round or turned over from how the gap sits, so the student has to turn it in their head. The working names the gap's shape and then shows the piece in place.

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