Students should estimate lengths throughout the course of this unit. One type of estimate is a judgment of whether an object is longer or shorter than some benchmark: Is your desk wider than 30 cm? How do you know? Is your arm longer or shorter than one meter? How do you know? Another type of estimate is a rough judgment of a length: Estimate the length of your foot in decimeters. Now estimate your foot in centimeters. Would it make sense to estimate the length of your foot in millimeters? Checking estimates by measuring will improve both estimation and measurement skills.
Ask students to draw a line that is two decimeters long. Then ask them to attach a 3 cm line and a 5 millimeter line to it. What is the total length of the line in millimeters? centimeters? decimeters? Repeat for other lengths.
What is 17 mm plus 4 cm? (It is not 21 anything.) Draw a 17 mm line and then attach a 4 cm line to it. Students can write their answer in mm and in cm. Repeat for other lengths.
