Suppose, you would like to divide the pie from the figure on the right-hand side into eight equal pieces. How often do you minimally have to cut? Every time you put the knife again in the pie counts as an additional cut.

It is possible to divide the pie into eight pieces by making only three cuts.
First cut a cross on through the top side of the pie. Then you'll have four equal pieces. Then cut horizontally through the middle of the vertical edge of the pie. You'll end up with eight equal pieces.