Homemade heating pad

[ebayer]

For Christmas, my daughter got me a homemade heating pad at a craft fair and I love it! The covering is flannel, and I don't know what it inside but I suspect it might be rice or some kind of food. I heat it up in the microwave for 2 minutes and it gives out a nice moist heat for almost an hour. I use it to warm up the bed before I get in it! So I don't have to cut it open to see what is in it, does anyone make homemade heating pads and if so, what do you put inside of them?

thanks in advance.

EDIT: Thank you for all the informative replies. I really appreciate it and am looking forward to making my own heating pads. I like the smell of jasmine rice as well, so I might start with that for the first project. Again, many thanks!