Silence

Scientific researches have shown that if a human being does not eat, will died. So there is no doubt that the mouth was made for eating.
Scientific researches also proved that if human beings do not have sex, the specie does not reproduce and finally disappear. So there is no doubt that the mouth was made for kissing.
But there is no scientific research that has proved that the mouth was made for talking. If a human being does not talk during his entire life, will not die for this reason.