In earlier lessons, students used rulers, measuring tapes, yardsticks, and meter sticks to measure lengths in inches, feet, centimeters, and meters. They learned that length units are represented on these tools as the length between tick marks. Students learned that if they line up the edge of an object with 0 on their tools, they can easily read the measurement from the tool without counting each length unit.

In this lesson, students extend their experiences with measurement to include situations when they can not use a tool to measure from 0. They know from prior experience that simply moving an object in space does not change its length (conservation of length). However, this understanding is challenged when students see an object that is not lined up with 0 on a measuring tool. Throughout the lesson, students are encouraged to make sense of and use equations that represent the measuring problems in each task (MP2). This understanding will support learning in future units with representing addition and subtraction on the number line.