Use a heat pack (wheat packs that you heat
in the
microwave oven work well), well - covered hot water
bottle, warm hand towel or a face washer wrung out
in hot water.
A
microwave oven heats unevenly, basically from the inside out, so it's possible that the formula could be very hot
in the middle of the
bottle but barely lukewarm on the outside where you can feel it.