A pun is a play on words; some puns involve a single word or phrase that has two different meanings.  Others involve two different words or phrases with the same sound, such as the words awl and all in a scene from Julius Caesar.  While puns are often humorous but can have serious purposes.

In the first scene of Act III of Romeo and Juliet, Mercutio retains his sense of humor even in death.  After he is fatally wounded by Tybalt and Romeo suggests that his wound is not too serious, he says,

No, 'tis not so deep as a well, nor so wide as a church door; but 'tis enough,'twill serve; ask for me tomorrow, and you shall find me a grave man.

The word grave means both serious and sad as well as also denoting the cemetery plot. After this line, Mercutio talks of a "cat" as having scratched him.  Here the word cat may be interpreted as meaning Tybalt, "The Prince of Cats" as Mercutio refers to him, and, of course, a real cat-like tear on him from the sword.  He also tells Romeo he is "peppered," which can mean both full of holes and angered.