Your rock could have received a final polishing under that waterfall, but the odds are it was already round when it came to rest there, says Chris Fouts, a geologist with the District Chamber of Commerce in Bancroft, Ont.

You're right, however, in assuming that water was the force that shaped it. Such a near-perfect sphere was most likely formed in a river "pothole," Fouts says. Potholes form when the eddies of a fast-flowing river generate enough energy to spin the rocks and gravel of the riverbed in a circular motion, eventually drilling a cylindrical hole into the river bedrock. More rocky material gets trapped in the hole, going around and getting more and more spherical, until some other natural event - such as the heaving action of frost - releases it from the pothole and sends it on further travels.

Vincent Vertolli, assistant curator of geology in the Earth Sciences Department of the Royal Ontario Museum, agrees that formation in a pothole is the most likely explanation for this rock. However, he notes other natural phenomena are also capable of creating amazingly regular spherical rocks. "Concretions" occur during the compaction of sedimentary rock, when the rock-forming materials are under enormous pressure, causing mineral-rich fluids to squeeze out. These fluids flow around any obstruction they encounter (such as a twig, a pebble, or a fossil) and the core begins to grow, somewhat the way a pearl grows in an oyster. Periodically these "pop out," to be found by surprised hikers, when the limestone weathers away enough to free them.