You stick them to your socks before putting on your boots. You shake 'em up and they warm up. You can put them under or on top of (or both) your feet and keeps them toasty warm for 6-8 hours.
www.warmers.com
I LOVE THEM!
You can get hand ones, entire foot ones (not worth it), for theraputic use ...
Wore them every day at Ridgway last year
... plus ... layers layers layers ...
http://www.aliciacharlton.com
http://www.mastersofthechainsaw.com/chainsaw_chix.htm