What are examples of puns?

Answer:

I recently spent money on detergent to unclog my sink. It was money down the drain.
Footnote:
In Shakespeare's Romeo and Juliet, Act III scene 2, the Nurse brings to Juliet news of Romeo's death. Juliet, not sure of the nurse's message, punning on "ay" (yes) and "I" (reference to herself) says,
What devil art thou, that dost torment me thus?
This torture should be roar'd in dismal hell.
Hath Romeo slain himself? Say thou but "ay,"
And that bare vowel "I" shall poison more
Than the death-darting eye of cockatrice:
I am not I, if there be such an "ay";
Or those eyes shut, that make thee answer "ay."
If he be slain, say "ay"; or if not, "no":
Brief sounds determine of my weal or woeAnd I say also unto thee, That thou art
In Matthew 16:18 of the Christian Holy Bible, Jesus says to Peter,
Thou are Peter, and upon this rock I will build my church...
The pun is on the name Peter and the reference to "rock." "Peter" is from Latin Petrus, earlier from Greek Petros, literally, "stone, rock."
If you prefer a less literary example, my favorite would be:
Q: What's the difference between a bird with two wings and a bird with one wing?
A: It's a matter of opinion.