This is called a 湯たんぽ in japanese. I have no idea what it’s called in english.
It’s something to keep warm in night time. (Go to google and type 湯たんぽ if you are interested.
You put hot water inside the stuff in the right (picture) and then you put in an extra layer of cloth on the 湯たんぽ.
The 湯たんぽ will warm your feet when your in bed.
Usually if your a human you can’t really sleep with out your feet warm.
So why not use this stuff that will last 8 hours……