Using a compass
draw a line AB.
Take a compass and draw an arc on AB in any measure.
Let the point where the arc cuts line AB be C.
Without changing the measure of your compass,cut the arc by placing the compass on C.Let that point be D.
Now,again cut the arc placing the compass on D.Let it be E.
Measure the angle EAB with a protractor.
You will find that it is 120 degree in measure.