Lisping is a problem producing certain types of speech sounds. People who lisp have difficulty pronouncing fricative (hissing) sounds, such as s or z. They may substitute other sounds or distort these sounds. For example, in the sentence "I see my sister," a person who lisps may substitute th for the s sounds and say "I thee my thithter." Another person who lisps may distort the s into a whistling sound instead of the normal hiss.