Geometry: using only compasses and straightedge
Draw a perpendicular to a line.
Draw a line AB. Choose a point X where you want to draw a perpendicular.
Choose another point Y below AB. Draw a circle with centre Y and width (radius) YX. It cuts AB at Z.
Draw a line through Z and Y to cut the circle at C.
Draw a line through X and C - you have drawn a perpendicular to AB at point X!