These bags have special built in temperature sensors. When thawing, feed after the blue sensor at the top has just begun to fade, this means it is the right temperature. It's very convenient as you don't have to check with a thermometer all the time. ****Description*****Brand New Ten BREAST MILK STORAGE BAGS with THERMAL SENSORTemperature sensor : The sensor is at the bottom and it shows proper time for feeding milk. If the blue color turns to white while red color stay as it is ,indicates that ready to feed milk (about 35 Celsius degree) If the blue and red color turns to white, it means too hot(about 40 Celsius degree) and do not feed milk and wait. *Material: pet LLdpe*BPA free and FDA approved material used - manufactured under strict hygienic regulations and standards in Korea. *Double zipper sealing - leak proof*Self standing*Easy sealing with zip lock. *Freezer safe (up to-20) *Pre-sterilized*Storing up to 200 ml*Name/Date/Time section at the top for easy recording. *Made in Korea - Customer quality guaranteed. - Authentic Brand*Can be use for Bottle Warmer These are specially designed for storing and freezing expressed breast milk. You can also use to store formula milk or baby food. So handy and not taking up too much space in your fridge/freezer. Then you can carry and use it on the go.