We chose to use 9 pieces of wood to make the shell. The math behind this is that esentially we are trying to make a circle. There are 360 degrees in a circle. we are using 9 pieces. two of the sides of each piece are being cut. 2x9=18, and then we divide 360 by 18. This turns into 20, so each side of each piece of wood will be cut at a 20 degree angle.