The teaser was:
Zona, the nutty professor, gathered the class one day for an experiment. Zona began, "I have in my hands a rock and a piece of wood. I am going to place them in this beaker of mysterious liquid, and you may observe what happens." Surprisingly, the rock floated and the wood sank! He turned to the class and said, "My question is: What is this mysterious liquid in the beaker?"
And the answer is: Water. : )
There is a rock, called pumice or volcanic glass, which will float in water.
There is also a wood that is so dense is will sink in water. It from the desert ironwood tree.