Lesson objective: Understand that measurement can be used to determine how much longer one object is than another.

Students bring prior knowledge of measuring to determine length differences from exploring standard units of measure in Grade 2. This prior knowledge is extended to estimating differences as students directly measure differences in lengths. A conceptual challenge students may encounter is not seeing the difference between the lengths of two objects, or being unable to accurately measure it.

The concept is developed through work with a ruler, which directly measures length differences.

This work helps students deepen their understanding of number because students need to either count units, or read numbers on a ruler.

Students engage in Mathematical Practice 3 (Construct viable arguments and critique the reasoning of others) as they explain which child in the lesson has the correct answer.

**Key vocabulary:**

- difference