A student measuring a lizard with calipers. A plastic bag keeps the lizard still and straight. The plastic bag also allows the students to obtain the lizard’s mass. Using a spring scale, students weigh the plastic bag without the lizard. This provides them with a pre-lizard mass. Then, students weigh the plastic bag with the lizard in it, which give them a with-lizard mass. By subtracting the pre-lizard mass from the with-lizard mass, the students can calculate the mass of the lizard.