How do you cut a cake into 16 pieces?

If the cake is circular cut from one side across the top through the middle to the other side. Either cut across halfway between the top and bottom of the cake, then cut at right-angles to the line to make a cross. Then cut across the cake halfway between the arms of the cake, then half way between the other tow arms of the original cross. The cuts will look like spokes in a wheel. Or, if you don't make the first cut, do all the other cuts and then cut between the lines again to divide the existing pieces in half.

A Square cake is easier. Halfway across one side of the cake cut across the middle of the cake from top to bottom. Cut across the cake halfway between the first cut and the outside edge of the cake, parallel to the first cut. Do the same to the other half. Turn the cake a quarter turn. Make the same 3 cuts at right-angles to the original cuts. The cake should have a checkerboard pattern with 16 squares.