Draw a rectangle on the whiteboard and tell the students that it represents one whole unit.
Draw a line down the middle of the rectangle and show them how the line divides the unit in half. While there is still only one unit, there are now two even parts to the unit.
Color in one half of the rectangle. Explain to the students that this rectangle represents one-half of the original unit.
Erase the rectangle and draw another rectangle, this time dividing it into three equal parts.
Color in two of the three rectangles. Show them how the dark area represents 2/3 of the original unit, while the light rectangle represents 1/3.