Articulation disorders may be attributed to a variety of causes. A child with hearing loss may not be able to hear certain phonemes pronounced at certain frequencies, or hear the error in their own production of sounds. Oral-motor problems may also be at fault, such as "developmental verbal dyspraxia" (a problem with coordination of speech muscles) or "dysarthria" (abnormal facial muscle tone, often due to neurological problems such as cerebral palsy). Abnormalities in the structure of the mouth and other speech muscles can cause problems with articulation; cleft palate, tongue thrust, and dental-orthodontia abnormalities are some common examples. Finally, it is difficult for children to hear and produce all of the different phonemes of a given language. Development is slow, and may take up to seven years. Sometimes, as children grow, articulation problems fade and disappear without treatment. Often, however, therapy is necessary. Treatment therapies may target semantic differences related to phonemic differences (e.g., teaching a child the difference between "toe" and "toad", underlining the importance of the final consonant), physical-motor differences (e.g., using a mirror to show a child the correct tongue placement for a particular sound), or behavior-modification techniques (e.g., repetitive production through prompts and fun learning games). Support and reinforcement of therapy practices, both in the classroom and at home, are crucial to the success of articulation disorder treatment (Boone 122-24, 259–62, 274–76).