Hold up the four containers of earth materials. Recall the last investigation, and establish that the containers hold the same volume of four different materials.

Were the weights the same?

Confirm that one of the materials was much "heavier for its size" than the others.

• Which of the earth materials was heaviest

• sand

• Which was lightest?

• organic soil

• And the two in the middle?

• mineral oil and water

As the students answer, place the materials in a line on the blank strip of paper. Put them in weight order, an equal distance apart. Encourage students to consult the data from their notebooks or class chart to determine "how much heavier." Discuss the way the containers are arranged and the way the data are represented in the class table.

• Right now the containers are arranged to show their weights from lightest to heaviest. But does this arrangement show how much heavier some containers are than others?
• Can you think of a way to arrange the containers differently, so that someone could tell, just by looking, which materials are close in weight and which are much heavier or much lighter than others?

Ask a volunteer to spread the containers along the paper strip to show how the weights of the materials actually compare. Do students agree with the result? Do others have different suggestions? Let them know that the data display they are creating is called a weight line.

• Is there anything we can do to show the data even better?

• write down the weights, mark intervals on the line, etc.

Introduce the investigation question:

What makes a good weight line?