This activity is a good warm-up to the study of the slopes of perpendicular lines.

Supplies:  Graph paper, protractors, rulers and scissors
                     Overhead projector to display sample graphs.  Display three 
                     (or four) graphs where the slopes vary, such as those shown 
                     below.

                    

Directions:
Form groups of three (or four) students.

Each student copies a different sample graph onto graph paper.

Cut the graph paper so that one of the endpoints is on the edge of the graph paper.

Find the slope of the segment.

Fold the graph paper so that the endpoints of the segment meets.

Unfold the graph paper.  Trace the folded line.  Find its slope.

Measure the angle between the original segment and the folded line.

Record the group's findings to show the slope of each segment and the slope of its fold line.

Make a conjecture about the slopes of perpendicular lines.

(Adapted from a problem in Integrated Mathematics 2 by McDougal Littell)