Answer: Nail moving towards a magnet
Explanation: Since the iron nails and pins move towards the magnet, it means that the magnet exerts a force on them. The force exerted by a magnet is called magnetic force.
I think you can only have 3 water molecules because you need 2 hydrogen molecules in every water molecule and you have 6 hydrogen molecules so 6/2=3 and the reactant that is limited would be hydrogen since it limits the amount of water molecules you can have