Why Do Some People Have Lisps?

What causes a lisp, and what are the different ways in which people overcome them?

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What is a lisp? (Speech Language Therapy)
"A lisp is a Functional Speech Disorder (FSD), and a functional speech disorder is a difficulty learning to make a specific speech sound, or a few specific speech sounds. The word 'functional' means that the cause of the disorder is not known. Indeed, in some literature FSDs are referred to as 'speech delay of unknown origin' or 'speech disorder of unknown origin'."