The Aqua-Relief Pads are cushioned with internal foam and are simply filled with hot water or microwaved before use.

The microwaving recommendations are that you fill the pad according to the instructions, heat for 1-2 minutes and feel to see if the temperature is to your liking.  If not, turn over and heat an additional 1-2 minutes and if it is still not to the desired temperature, heat in 30 second increments until you have reached desired temperature.