Geometry: using only compasses and straightedge
Draw a line perpendicular to the end of a line.
Draw a line AB from A to B. We are going to draw a line perpendicular to the end "A".
With your straightedge extend one end to X.
Put yor compasses on A. Make a them less wide than AX. Make them go round in a circle and mark line XAB where they cross at Y and Z.
Center your compasses first on Y then on Z and draw two circles like this diagram.
Draw a straight line MN through the points where the two circles cross.
Line AN is perpendicular to the end A of line AB. Success!