PHYSICS

Drop a small pebble into a pond and it vanishes in seconds. Launch a steel ship weighing thousands of tonnes and it rides on the surface. Both are made of dense material, yet one sinks and one floats. The secret is not the metal alone but how much water the object pushes aside. When anything enters a fluid, the fluid pushes back upward. That upward push is called buoyancy. If the upward buoyant force is strong enough to balance the object's weight, it floats. If not, it sinks. Understanding this puzzle takes us back to ancient Greece and one of history's most famous scientists.