A lisp is a speech impediment, historically also known as sigmatism. Stereotypically, people with a lisp are unable to pronounce sibilants, and replace them with interdentals, though there are actually several kinds of lisp. The result is that the speech is unclear. "Interdental" lisping is produced when the tip of the tongue protrudes between the front teeth and "dentalised" lisping is produced when the tip of the tongue just touches the front teeth.
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