A ruler is a tool for measuring length in standard units. In the U.S. customary system, the inch is the basic unit; the metric system uses the centimeter. To measure correctly: align the zero mark of the ruler with one end of the object, then read the mark at the other end. Standard units allow everyone to communicate measurements consistently, unlike nonstandard units.
Have students measure the same object with both an inch ruler and a centimeter ruler and compare the numbers — this reveals that the unit size affects the count. Practice aligning the ruler at zero, not at the ruler's edge. Include objects that fall between whole-inch marks to discuss the need for smaller units later.
You already know how to measure length using nonstandard units — lining up paperclips end to end, or counting how many blocks long a desk is. That approach works for comparing two objects in the same room, but imagine trying to tell a friend across town how long your desk is: "seven paperclips" only makes sense if they have identical paperclips. Standard units solve this problem by giving everyone the same reference length.
An inch and a centimeter are fixed, agreed-upon lengths that mean the same thing everywhere. Because everyone uses the same definition, "the desk is 24 inches long" communicates precisely, whether you say it locally or anywhere in the world. Centimeters are smaller than inches — roughly the width of a fingernail — which is why measuring the same object in centimeters always gives a larger number than measuring it in inches. More, smaller units are needed to cover the same distance.
A ruler is essentially a number line for length. The zero mark is the starting point, and every tick mark is one unit further along the scale. The most important habit is aligning the zero mark — not the ruler's physical edge — with one end of the object. Many rulers have a small blank border before the zero, so placing the ruler's edge flush with the object would produce an error from the very start.
Once the ruler is aligned, read the mark that lines up with the other end of the object. If the end falls exactly on a whole-number mark, that is your measurement. If it falls between marks, you can round to the nearest whole number or, with a finer ruler, read a fraction of a unit. The result is only as accurate as your alignment — a sloppy start produces a wrong answer no matter how carefully you read the scale at the other end.