The answer to the crossword is 'EARS'. Ears are organs of hearing and balance that are located on either side of the head. They are responsible for detecting sound waves and transmitting them to the brain, which interprets them as sound. They may also help to maintain balance by detecting changes in the position of the head. The phrase 'they may perk up' could refer to the ears perking up when they detect a sound, or it could refer to someone's mood being lifted when they hear something pleasant.