Here's a tutorial for those of you that want to make one for yourself or others.
1 kitchen towel (I picked one from Walmart for 98 cents)
1 bag of rice (not instant)
Fold and cut in half.
Turn inside out with the wide opening as shown.
Fill about 1/4 of the bag with rice. You do not want it too full as remember it is going to need to lay on a knee or some other body part!Stitch close. You now have a heating bag. Heat in a microwave for 2 minutes or however hot or cool you like it. Be careful if you use on kids (of course)!.
I have found Terry cloth kitchen towels work the best and are the best shape. If you want smaller bags you can cut in fours and follow the same directions. They could be used to warm up chilly hands , feet, etc!! Below is a picture of the one my husbands coworker made me years ago. She even included a small carrying bag!!