Answer: Pumice

If you’re planning on floating an S.O.S. message off the island you’re stranded on and you’re short on bottles and coconuts, you could always attach it to a big hunk of pumice–the only rock that floats.

Pumice has an extremely low density, so low in fact that a given volume of it is 1/12th the weight of granite, 1/45th the weight of lead, and–most relevant–1/2th the weight of pure water. Drop a pumice stone in the ocean and it will make a splash like any stone, but then it will float away on the waves.