Birthday cake, a sweet brain test

Now Briquinex takes you to a birthday party in order to test your brain. At this party, one of the boys comes up with a challenge to spice up the fun. He asks who can cut the birthday cake into exactly 16 pieces, but by making only 5 straight cuts, and you can't move pieces of the cake as you cut. How will you solve this Briquinex test?

foto source:wikipedia