I'll second that.
Tried to use some that were expired a few weeks ago and they didn't even get a little bit warm.
They were WAY out of date, but I expected at least a little warmth... and got nothing.
Always had great results with non-expired ones though.
Also, geez... 20 years ago, or more... I had a hand warmer pack that was about the same size as a hot hands pack, but thicker. It was flexible clear plastic with clear liquid and a little metal disc inside. Whey you flex the disk it would activate the warming. As the pack expended heat the clear liquid inside would crystalize.
Dropping the pack in boiling water would "recharge" it and make the crystal convert back to liquid.
It was an awesome, reusable little bit of kit.
Anyone know is there is a reusable product like that on the market today?
You can still get them. They only have about a 4 hr heat time at most, from what the ads say. Google "reusable handwarmer"