An exploratory dig in Central Africa was not going well, until one of the students found a small set of rocks which moved curiously, and when put into a box with other rocks would seemingly absorb them and grow. In fact,
if several of these rocks were left together, after a few weeks, there would be
pebbles.
The student studied them for a while, then took them to his professor
and explained what had happened, saying, "Doctor! Living stone, I presume."