I have been making these for a long time. I started out using soy beans but they scorch easy. The rice makes a moist heat. Now I usually use a combo of rice, lentils, flax and lavender. I love the lavender smell and it is supposed to be soothing. I grow my own lavender so it is "cheap" and I can use as much as I like. I even sewed darts in 2 so they fit on my knees.