Purpose of the discussion

The purpose of the discussion is for students to be able to explain how to measure the volume of rocks and other solid objects, and understand the advantages of using displacement to determine volume. Again, focus the discussion on the investigation question.

Engage students in the focus question

To engage students in the discussion, first allow time for them to complete the Reflection Questions in their notebooks [Volume reflection], and then return to the investigation question:

How can we measure the volumes of rocks?

• If you wanted to find the volume of your hand, would you use cubic centimeters to build a replica or use displacement of water? Why?

If there is time, ask students to consider the following question:

Do you think the displacement method for measuring solid volumes would work if we used mineral oil instead of water? Why? (Yes, any liquid will work.)

Let students know that it is often difficult to estimate volume, just as it can be difficult to estimate weight. Practice helps, but measurement is better. Unfortunately, there is no "volume scale" like a "weight scale." For irregularly shaped objects, we can measure volume only indirectly, by measuring water displacement, as the students did today.

Summarize the discussion and recap the investigation

Use students' own words to summarize how can we measure the volume of rocks and other objects using displacement.

As you review the investigation, make sure students understand the following main points:

• Volume is often measured in cubic centimeters.
• With practice, we can estimate volumes by comparing new objects with objects whose volumes are already known.
• The volume of any solid objects can be measured by the displacement of water.