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I think sodium metal would be a good thing to have. It is much like rubidium (the metal which explodes on contact with water) but sodium take a few seconds to explode, and the hotter the water the bigger and quicker the explosion. I don't think it needs to go into that much detail, but just make it take a few seconds till it ignites. By the way it would be a powder, unlike rubidium. Thank you for reading.