If the bottle has been in the refrigerator, run warm tap water over it or place the bottle in a pan of hot (not boiling) water. Shake the bottle occasionally while warming. Warming time should be less than 15 minutes. Test the formula temperature before feeding. It should not feel warm or cold when dropped on your wrist. Do not use the microwave to warm formula. Serious burns may result. Actually, most babies don't seem to mind whether their bottle is warmed or straight out of the refrigerator. Some may prefer a consistent temperature from one feeding to the next. Once a bottle has been warmed, keep it for no more than one hour. Then discard it.