I bought this for my son's school backpack, but he doesn't like it, so I'm selling it. It's only been tried on. I bought it online for about 1400 yen. It freezes naturally below 28°C (82°F). It can be easily attached to the shoulder strap with the included hook. Comfortable for business backpacks too. Also for school backpacks, rucksacks, strollers, and car seats!