Black

At an university, a lecture about trees is in progress. The lecturer explains
that different trees produce different kinds of nuts. Some of the nuts are round
shaped, some angular in shape, some are green, some are brown etc. Only one tree
in the world has black nuts. Professor names that tree in Latin. Then Professor
notices that one female student is not listening to him, but is fully absorbed
in reading a book. He approaches her, and, pointing a finger at her, shoots a
question, "Whose nuts are black?"
Startled, the student replies, "Those of blacky."