If you take 25 grams of iron filings or powder and you mix it with one gram of sodium chloride (table salt) in a ziplock bag then shake it to mix it. then you add a tablespoon of charcoal or sawdust and shake to mix again. seal this in an airtight jar or else the iron will oxidize. to activate, put the amount you want in a ziplock bag and ad about a teaspoon of water. squeeze and shake the mixture until it gets hot. ours got extremely hot and if you get the amounts of chemicals perfectly right, it'll last a few hours.