Geometry: using only compasses and straightedge
Draw a line with the straightedge.
Draw 2 perpendicular lines to the line
anywhere on the line. (How?)
Centre the compasses at where the perpediculars meet the line, keep the compasses at the same width and draw two equal circles.
Draw a straight line through where the circles meet the perpendiculars. This line is parallel to the first line. When we drew perpendiculars we made a rectangle, and opposite sides of a rectangle are parallel.
Join the two lines at any four points A,B,C,D and you have a parallelogram ABCD.