A lullaby is a soothing song or piece of music that is usually played for (or sung to) children (for adults see music and sleep). The purposes of lullabies vary. In some societies they are used to transmit cultural knowledge or traditions. In addition, lullabies are often used for developing communication skills, indicating emotional intent, maintaining infants' full attention, modulating infants' arousal, and regulating behavior. Perhaps one of the most important uses of lullabies is as a sleep aid for babies. As a result, the music is often simple and repetitive. Lullabies can be found in many countries and have been around since ancient times.