Construction problems with compass and straightedge
Construction problems use only an unmarked straightedge and a compass. With the straightedge you may draw an arbitrary line, a line through a given point and a line through two points. With the compass you may draw an arbitrary circle, a circle with a given centre, a circle with radius equal to a given segment, and lay off a segment equal to a given one on a ray from its origin. Measuring with the millimetre marks of a ruler, or drawing parallels with its two edges, is not allowed. A construction consists of finding the steps and justifying that the resulting figure satisfies the conditions. The compass point is sharp: handle it carefully and under adult supervision.
“Allowed or not”: the teacher names an action (“measure in millimetres”, “draw a line through two points”) and pupils say whether construction rules allow it.