A sore on a police officer's foot is called "corn on the cop." This is a pun on the phrase "corn on the cob," with the word corn referring to the callus that often appears on a person's foot.
This question is a good example of a joke or riddle with the punch line hinging on a person's understanding of the pun. Another word for pun is paronomasia, and it is simply a form of word play. The words used in a pun have multiple meanings. Sometimes the words used in a pun have similar spellings or sound the same.
The answer to this riddle uses both concepts. The word "corn" has multiple meanings; it references a sore on the foot as well as the vegetable in the answer. The word "cob" is a play on the word "cop," referencing the police office in the original question.
Many people consider puns to be the lowest form of humor. However, puns were used by Shakespeare and even in the Bible. In "Romeo and Juliet," for example, Mercutio says, "Tomorrow...you shall find me a grave man," after he is fatally stabbed. One of the oldest puns in the world is found in the Bible, in the Book of Judges. This passage states, "Thirty sons rode around on 30 burros and lived in 30 boroughs."