I was skeptical of how well this very moderately priced humidifier could work. The alternative that was suggested just rattled around in the guitar, however, so this seemed like a better choice, if it worked. Lo and behold, it does. It is deceptively simple. You squeeze the sides, and out pops a sponge. You could do your dishes with it, and I assume that if you want to replace it, a chunk of kitchen sponge will do the trick. You soak the sponge to your liking, pop it back in the unit, and snap the top back on. Now for the feature that I think sets this apart from those plastic tubs. The unit slides between the strings in your soundhole. The little tab that you can see in the center bottom of the picture (which is the top when you place it in your guitar) is actually for the string to snap over, so that the humidifier cannot slide or come loose. The great thing about this is that it never touches your guitar (other than the strings) or the inside of the body, and cannot rattle around and make noise or damage the inside of the sound chamber. You can flip guitar all around, and the unit stays put. Also, as long as you have not totally sodden the sponge without squeezing it out, no water can come out to mess things up. So it does what it is supposed to, without creating any cause for worry.