Introduce the investigation question:

Same weight, same volume?

Wait — that question seems very familiar. Hasn't the class already done this investigation? No, not exactly. Draw attention to the investigation question for the first session of this unit:

Same volume, same weight?

Is this the same question? How is it different? Can the class guess what today's investigation will be?

Ask a volunteer to summarize the investigation the class has already done weighing samples of earth materials that have the same volume. As a reminder, show one set of the containers holding equal volumes of sand, organic soil, water, and mineral oil.

• What did we discover?

• When volumes are the same, different materials can have different weights.
• Some materials are heavier for their size than others

Explain that today the class will explore the opposite question.

• When weights are the same, how do the volumes compare?
• How might we go about finding out?

Listen carefully to the answers. Make sure students are clear on the difference between weight and volume, that they can imagine how to turn the previous experiment around, and that their preliminary thinking is on track.