Time flies like an arrow.
Definition: fly (verb) – to move through the air or through outer space.
Like (preposition) – having the same characteristics or qualities as; similar to.
Fruit flies like a banana.
Definition: Drosophila melanogaster is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly.
Like (verb) – to feel attraction toward or take pleasure in.
Puns are jokes that use words that have different possible meanings or words that sound alike but have different meanings (called homophones).
In order to understand the joke, you need to be aware of both of the meanings that the word(s) can have.