Whilst going through [one] my junk drawers tonight, I happened upon 2 yellow 'starfish-shaped' objects, [I bought 'em about 5 years ago, at the onset of winter, from Aldi]. This is what is printed on 'em:-
" Bending the metal plate activates your pocket hand warmers. This causes the liquid inside to crystalise. Depending on the outside temperature, your pocket hand warmers will give up to 55C [degrees] heat for approx. 1 hour. Before using the pocket hand warmers again put them into boiling hot water until the contents are completely clear and fluid ( approx. 10 min ). Wrap the pocket hand warmers into a towel for optimal protection during/after boiling.
Attention - Hot: Let the hand warmers cool before picking them up or moving them! The pocket hand warmers are now ready for you to activate again. "
I have just bent the little bit of metal inside the warmers, and after ten seconds, it was very hot.