The Castelli Active Cooling functional underwear uses a special open knit made from hydrophobic and hydrophilic yarns. This retains a small amount of moisture on the body, which can evaporate thanks to the free airflow. As a result, your skin is cooled directly. It is best worn under particularly cool Castelli jerseys, such as the Climber's or the Superleggera – or under any other breathable jersey.