Ask students if they have ever been concerned that they bought something that did not live up to its promise.
In a relevant story, Archimedes of the ancient Greek city of Syracuse was called upon by the king because he was worried about a similar problem.
Discuss image one: the king had given the goldsmith one bar of gold with which to create a new crown. However, when the king gets the finished crown he grows concerned that he's been cheated.
Discuss image two: Archimedes presents an idea to the king. Obviously the king doesn't like the idea, but even so have the students explain why Archimedes might recommend melting down the crown.
- to compare the volume of a gold brick used to MAKE the crown to a gold brick MADE FROM the crown