A smile is a face made by flexing the muscles near both ends of the mouth. The smile can also be made through the eyes (See 'Duchenne smile' below). Smiles usually express happiness. A smile can be natural or fake. However, smiling can be different with animals. When smiling, the teeth shows, but sometimes animals do this when they are threatening. When chimpanzees show their teeth, it can also be a sign of fear.