Toe warmers are well-known to northeastern skiers (especially here at brutally cold Sugarloaf in January)! They often don't really fit in ski boots if the boots truly fit properly. However I often stick them in the top of the boot in front of the shin. Maybe it's all in my head, but I think that the warm blood makes it to my feet! Placed directly on the feet, even thru a sock, they have been known to get too hot. For biking, they can also be placed on the ankle and held in place with a velcro strap.