This document discusses tools used for measurement in science class. It focuses on metric rulers and how to accurately measure length in centimeters and millimeters. Key points include: - Measurements are observations made using tools like rulers, while calculations involve manipulating measured numbers with formulas. - When calculating, measurements must use the same units (e.g. cannot mix cm and mm without converting). - When measuring with a ruler, start from the black line not the edge, and read measurements between the black lines in mm up to the next cm line. - Mass and weight are different - mass is the amount of matter, while weight depends on gravity.