Abstract: The present development is a liner unit comprising a light-weight polymeric material that becomes cold when exposed to water, which can be used to line a soft-crown cap, such as a baseball cap. The liner is sized to fit snuggly within the cap, and may be reversibly attached to the interfacing of the cap. Alternatively, the liner may comprise a magnet, sized and oriented on the liner so as to allow the magnet to engage with the underside of a button secured to the cap.