BootBurners take advantage of natural ingredients to hold and disperse heat in an easy and safe fashion. A few minutes in the microwave are all it takes to warm them up nicely. Upon inserting them into a boot they immediately begin to radiate their heat into the boot itself, permeating every corner with warm, luscious warmth. Constructed of two parts, a washable outer sleeve and an inner ?burner? bag, BootBurners feature tough construction mixed with soft, attractive fleece. The outer sleeve serves three distinct functions; it removable for easy washing (important!), it protects hands from the hot ?burner? bag, and it helps fill the boot?s dead air space and thus helps the BootBurner heat more efficiently. The inner sleeve holds the heating material and allows a high degree of breathability to ensure the maximum flow of heat possible. Life expectancy for the ?burner? bag is between 50-100 uses although some may last longer. When used up the bag is easily replaced with a new one, available online or in stores everywhere. Unlike other boot warmers that simply warm dead air space or are focused entirely on warming the sole of the boot, BootBurners heats the entire boot. And, the patent pending design fills every part of the boot ensuring virtually no dead air spaces. By transferring their heat directly into the boot itself BootBurners provide a more thorough heating than most any other heater available. BootBurners goes way beyond warming just the surface layer of your liner or simply heating the sole of your foot, the ?Burner?s heat penetrates the entire boot warming and softening everything in it?s path. And ?no?, properly heated BootBurners won?t harm custom liners or insoles. Of course, if you go nuts and really heat them up, getting them really, really hot, then yes, you could damage both your liners as well as the Burners. BootBurners are powerful, go easy with them until you have a feel for how they work.