Info from Manufacturer: It is important to keep your valuable instrument at the proper humidity level. Most experts agree that this is between 40% - 50% humidity. Too low and your instrument finish can be damaged and the wood can crack. Too high and your instrument will swell, fret ends become sharp and it will not sound right. The GROVER® Ukulele Humidifier is the perfect product to combat these problems and to protect your expensive instrument. Our unique cover allows you to partially… or completely cover the sound hole for extreme conditions and allows you to add water without removing the cover. For very moderate conditions, you can do without the cover and hang the humidifying tube between the strings. Made of soft latex, the GROVER® Ukulele Humidifier is harmless to all instrument finishes. • Humidity indicator included.