One of my friends used to have a lisp when she was little. Some of you may not know what a lisp is; it’s a speech disorder (‘sigmatism’) that usually involves people having difficulty producing sibilants and replacing them with interdentals or dentals resulting in unclear speech. Lisps are often down to physiological causes, such as deformations, a large, bruised, or swollen tongue and underbites.
Treatment (speech therapy) can take months or even years. I just came across a relatively recently developed instrument to support speech therapy, the speech buddy: