I have a moderately bright group of year 9 students who usually pick up any new topics fairly comfortably. All fine until we got to geometric construction (perpendicular and angle bisectors, perpendicular to and at a point). Somehow, they just did not grasp this. I need to come back to the topic and try again but I cannot work out what they don't understand. Any ideas. What makes this topic harder than everything else we've looked at?