TONGUE TWISTER OR PUN?

 

While making my bed this morning, and due to the strange workings of the brain, I started humming  a little poem I learnt in my early youth:

A Flea and a Fly
A flea and a fly in a flue
Were inprisoned.
What could they do?
"Let us fly!" said the flea.
"Let us flee!" said the fly.
So they flew through a flaw in the flue.

I have done some research on this, and it passes off as a tongue twister. It is not. It is a pun riddle. Flea vs. flee; flue vs. flew; fly noun vs. fly verb. It is a good example of the workings of puns in the English language. My dictionary defines "pun" (retruécano) as "the humorous use of a word or phrase to emphasize or suggest its different meanings or applications, or the use of words that are alike or nearly alike in sound but different in meaning; a play on words."