This experiment will astonish your kid and will make you a hand warmer that will last up to 6 hours.
You will need:
A pencil
A pencil sharpener
A self-sealing sandwich bag
1 teaspoon of salt
2 tablespoons of water
2 tablespoons of vermiculite that you can find in a garden store
2 tablespoons of iron powder
2 tablespoons of pulverized activated charcoal that you can find in a pet store
Let your child sharpen the pencil and crush its shavings until you have about 3 tablespoons of pencil shavings. Mix all the ingredients in a bowl except the water then put them in the sandwich bag. Add the water to it and seal the bag making sure that some air entered inside. Agitate the bag! And voila the mixture will start to heat up!
If your child is in his school age, you can explain to him that this mysterious heat comes from a chemical reaction between the ingredients you used!