Microwave with safety.
This Hottie solution uses water locked into a plastic case, so no "gel" queries, or dangers of overheating and setting on fire as with wheat bags. The worst you can do is (eventually) to cook tiny holes into the plastic, but my last one had 1 year's constant daily use before a tiny leak appeared. (fixed until replacement with Duct tape) The warmth lasts for up to two hours, and even longer in a warm environment where your own body heat will help to maintain warmth. I use it as a body wrap for a bad back and hip problems, or in a different case as a night time hot water bottle. It can also be worn under a top, and not show, since it is slimline, so you can have comfort and fashion together. Follow the heating instructions carefully for best results, and always use an insulated cover. Handle with care after heating as it will be Hot.
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