The humidifier that let's you see when it's ready to refill. Oasis works by allowing water to evaporate through the fabric. Unless the cap is opened, outside air cannot get in to replace the evaporated water. The vacuum created by this process causes the fabric to shrink inward indicating that it's time for a refill with the included refilling syringe. It has two small neodymium magnets located inside the seam making it easy to attach to your case by using the included clip or sticky back steel strip. It works with any string instrument case.