A soft rubber container
or hot water
. Designed to be leak proof
when possible; they look
like a tiny warm pink pillow
According to familyeducation.com's contributing writers: You could place them on baby's bed prior to rocking and night feeding. This can prevent that Cold Sheets recoil "Waaaaaaa!" phenomenon (Mom and Dad's arms and lap are so much warmer than winter sheets). Remember to remove the bottle and put it out of baby's reach.
It's so common artists have use it to compose art: See the Andy Warhol Hot Water Bottle (http://store.yahoo.com/whitneystore/anwarhotwatb.html).
Hot water bottles are good for aches and pain, cramps, flu, and arthritic pain. Primarily they keep you and your loved ones warm.