Students compare the lengths of objects using language like 'longer,' 'shorter,' and 'about the same.' They measure using non-standard units and compare results, developing foundational skills in measurement.
You already know how to compare lengths by looking — you can tell that a pencil is longer than an eraser when they're right next to each other. But what happens when two objects are far apart? Or when you need to compare more precisely than "longer" or "shorter"? This is where measuring comes in. Measuring means using a unit to count how long something is, so you can describe and compare lengths with numbers.
A unit is whatever you decide to measure with — a paper clip, a crayon, a finger. When you lay paper clips end to end along the side of a book and count 7 paper clips, you've just measured the book as "7 paper clips long." Now you can compare the book to a box without putting them next to each other: if the box is 4 paper clips long, the book is longer. Numbers let you compare things that aren't in the same place.
The most important rule when measuring is to use the same unit for both objects, and to line the units up without gaps or overlaps. If you measure the pencil with big paper clips and the eraser with small paper clips, the numbers don't mean the same thing anymore — you can't compare them fairly. This is called using a consistent unit, and it's the reason measuring works.
Here's something interesting to notice: the smaller your unit, the bigger your number. If you measure a table with your hand, you might get 10 hands. If you measure the same table with a paper clip, you might get 60 paper clips. Both answers are correct — but the unit is different. When you get to rulers and standard inches and centimeters, you'll use the same idea: agreed-upon units that everyone uses the same way, so that measurements can be compared across different people, places, and times.