Now that you know that drying socks in a microwave can cause all kinds of problems, you may wonder if it's safe to put a sock filled with rice in the microwave to make a heating pad. Fortunately, heating a rice-filled sock is safe. Make sure the sock you use is 100% cotton so it won't melt, and heat it on high for 2-3 minutes. Don't leave it in longer than 3 minutes as it can burn your skin.