Is important for students to understand that numbers in science often involve measurements and that measurements have limited precision. The two activities described in this article demonstrate the limitations of measurement tools and explore the varying degrees of precision associated with the tools. Teachers should expect students to read measuring devices to the smallest units marked on the equipment. If a ruler is marked to the 0.1 cm, then students should read it to the 0.1 cm. Estimating the next digit should only be expected after students have mastered the first step. The variation in a set of class data will depend on the students and the equipment. With teacher guidance, students should examine the range of variance and agree on an acceptable range. Students whose data fall outside the range should be able to repeat their measurements without penalty.