Archimedes was a Greek scholar who came up with the idea of displacement in his bathtub. He was so excited that he yelled "Eureka!" and ran through town naked to tell everybody.
Make your predictions. Will the items sink or float?
Many years ago people accused of being witches were thrown into the water as a test. Float or swim and you were a witch. Sink and die and you were not a witch. Dumb but true. Really, look it up!