Do not hand in the piece of bread.

[thinpaperwings]

At the end of each chapter in my differential equations textbook are labs that the professor can choose for the class to turn in. Lab 1.2 examines Growth of a Population of Mold. It involves putting a soggy piece of bread in a sandwich bag in a warm place, and tracking the growth of mold on the bread over the course of two weeks. At the end of the description of the lab, in the section that describes how to write up your report, is:
Do not hand in the piece of bread.