The devices are about the size of a stack of two quarters and stick to your skin (typically your stomach or tricep) with a strong adhesive. When you apply the CGM, a flexible filament only a few millimeters long and as thick as dental floss embeds just under your skin. Enzymes on the tip of the filament interact with glucose molecules in the fluid between your cells; this chemical reaction then produces an electrical current, the strength of which can be translated into a glucose concentration number you can see on your phone, usually expressed in milligrams per deciliter or mg/dL.