Kala recommends the MusicNomad Humilele Humidifier for humidity drops below 40%. The ideal safe zone for a ukulele is an environment with 45-55% humidity. Tonewood needs a certain amount of moisture to avoid complications that can occur from dry wood. Dry wood can warp, shrink and worse yet, crack your instrument! Wood dryness can occur due to air conditioning during the summer and with indoor heating during the winter. That's where the Humilele comes in! The Humilele rests securely on top of the strings to safely and evenly hydrate your Ukulele. Simply soak MusicNomad’s Humilele sponge in distilled water, replace the top, and position it between your ukulele strings. Very easy and low maintenance. Fits all sizes.