What makes a good weight line?

Students evaluate four weight lines in their science notebooks [Which weight line?].  The work is presented through a concept cartoon involving five characters. One of them, Darwin, a dog, has four dog biscuits. The biscuits are all the same size but have different weights: 5 grams, 10 grams, 20 grams, and 100 grams. Darwin wants to arrange the biscuits on a weight line so he can see how the weights compare. In the cartoon, four characters show Darwin their weight lines.

Ask students to place the four biscuits on each character’s weight line. When they are done, have them talk in their groups about the strengths and weaknesses of each weight line and then write a letter about them in the notebook [Dear Darwin].

In fact, three of the weight lines are problematic; only Fern's is well constructed. As you circulate among the students, see if they are noting some of the difficulties — improper endpoints, erratic intervals, missing units of measure, etc. — but do not comment on them at this time.

• What makes a good weight line?