Common Sense: Wood floats - you can make boat with wood. Iron sinks - you cannot make boat using iron.
Observation: Everyone today knows that ocean going streamers/container ships are made of steel.
Science behind: Archimedes principle of flotation by Archimedes of Syracuse (287–212 B.C.), Greek mathematician and inventor. The upward force (flotation force) is equal to the volume of liquid displaced. By making a ship with steel hulk with large volume, the ship can make to float.
Stupidity: In the thirteenth century, Christian monks had ripped apart Archimedes’ original manuscript, washed it, and covered it with religious text.