How do you fold a paper into 3 equal sections?

Folding a sheet into thirds is the process used for placing letters into envelopes.

Bring the bottom of the sheet up toward the center, but do not crease. At the same time, bring down the top of the sheet over the bottom. As the sections roll past each other, there is one position where they form a stack of three equal areas. You can crease at this point to make the shortest possible folded sheet.