These are my apples.  The first one (top) is a plan drawing, showing a bird's eye view of the apple.  The next one is a section drawing of the apple if it was cut horizontally.  The final one is a section drawing if the apple was cut vertically.  Since I am not the best artist, I think these exercises help a lot.  We had to measure the apple with a caliper to make sure our drawings were exactly to scale.