These are easy to use, cost effective way to heat detect. ESTROTECT works like a scratch-off card. When cows have been mounted, the detector's silver surface is rubbed off by the friction of the mounting and will begin to show the color underneath. With each mount, the surface will gradually turn from silver to its indicator color indicating a true heat. Approximately 50% of the silver scratch-coat should be removed to indicate a true heat. Buy by the sheet, which has 5 detectors per sheet.