The heat sock 6.1 from Lenz ensures warm feet on cold days. The knee-high socks are super elastic and cuddly and their battery-operated, three-stage heat function keeps feet and legs at a pleasant temperature. No cold feet - in all weather conditions. The slim lithium packs (not included in the scope of delivery) can be attached to the sock cuff - barely noticeably below the knee - using press studs. It's bitterly cold outside. No problem with the heat sock 6.1. it stays warm from below.