All in a day's work

Luke teaches an English conversation class to freshmen--many of whom are speaking English for the first time. Recently, he had a class that required the students to list various objects made of particular materials. Desks, pencils, and doors are made of wood; windows, glasses, and jars are made of glass, etc. He wasn't quite prepared for the answer when he asked the question: "What is made out of leather?" More than one student quickly said, "Pigs!"