Yes, you heard right, Heat Wave from Red heart actually self activates to keep you warm while you're outside this winter. 

A revolutionary yarn that is activated by sunlight, even on a cloudy day, to keep you up to 12°F warmer. 

Upgrade your outdoor gear and stay warm at sporting events, everyday outside activities or even walking your dog. 

Worsted Yarn
100% Acrylic
198 Yards (181 Meters)
Skein Weight: 3.5oz (100 g)
Gauge
Knit: 18 stitches x 24 rows = 4" (10cm) on US8 8 (5mm) knitting needles
Crochet: 13 single crochet x 14 rows = 4" (10cm) on US H8 (5mm) hook
Care: Machine Wash and Dry

How do they do it? I don't know but try these fun accessories and test it out for yourself.