Time flies like an arrow; fruit flies like a banana
From Wikipedia
"Time flies like an arrow; fruit flies like a banana" is a humorous saying that is used in linguistics as an example of a garden path sentence or syntactic ambiguity, and in word play as an example of punning, double entendre, and antanaclasis.
The point of the example is that the correct parsing of the second sentence, "fruit flies like a banana", is not the one that the reader starts to build, by assuming that "fruit" is a noun (the subject), "flies" is the main verb, and "like" as a preposition. The reader only discovers that the parsing is incorrect when it gets to the "banana". At that point, in order to make sense of the sentence, the reader is forced to re-parse it, with "flies" as the subject, "fruit" as a qualifier of "flies", and "like" as the main verb.
The role of the first sentence is to predispose the reader towards the incorrect parsing of the second one. However, after re-parsing the second sentence, the reader presumably becomes aware that the first sentence too could be re-parsed in the same way.
The sentence "time flies like an arrow" is in fact often used to illustrate syntactic ambiguity.
Modern English speakers understand the sentence to unambiguously mean "Time passes fast, as fast as an arrow travels". But the sentence is syntactically ambiguous and alternatively could be interpreted as meaning, for example:
(as an imperative) Measure the speed of flies like you would measure the speed of an arrow—i.e. (You should) time flies as you would time an arrow.(imperative) Measure the speed of flies using methods that an arrow would use—i.e. (You should) time flies in the same manner that an arrow would time them.(imperative) Measure the speed of flies with qualities resembling those of arrows—i.e. (You should) time those flies that are like an arrow.(declarative) Time moves in a way an arrow would.(declarative, i.e. neutrally stating a proposition) Certain flying insects, called "time flies", enjoy an arrow.
In addition, the sentence contains semantic ambiguity. For instance, the noun phrase "Time flies" could refer to all time flies or particular time flies, and "an arrow" to all arrows, a particular arrow, or different arrows for different flies; compare "Fruit flies like a banana", "Fruit flies ate a banana", "Fruit flies live on a banana". Moreover, "Time flies" could refer to "flies of the Time magazine", or "flies of the Pink Floyd song Time". Furthermore, "like" as a verb could either signify general enjoyment, or the usage of a like button.- END -