What would you like to measure?
Mass:
Mass is often confused with and used the same as weight. Weight invloves the gravitational pull on the object. Mass will be constant no matter where you are, your mass is the same on earth as it is on the moon. But you will weigh less on the moon because there is no gravity pulling on you to make you weigh more.
American:
Ounce-oz
The ounce is a small unit of measuring mass/weight. This paper clip would be most accurately measured using ounces.
Pound-lb
Animals, such as this fox, would be weighed in pounds. When something is more then 16ounces, they are weighed in pounds instead so that the number is smaller. It is usually best to use the largest unit of measure inorder to keep the numbers lower.
Tons-t
Cars and pick-up trucks are measured in tons. A ton is 2,000 pounds. imagine how heavy a truck is. If a truck is 2 tons, that's 4,000 pounds. What is easier to write or say? 2 tons right? So that is why tons are more commonly used when discribing heavy objects!
Metric:
Gram-g
Quarters can be measured in grams. Grams are a small unit of measurement. One paper clip is about the same as one gram. How many grams do you think a quarter weighs? How many paper clips do you think are the same weight as the quarter? There are scales that are designed to compare the weights of two objects. They are nice when you are comparing, but do not always give you the exact weight/mass of the object.
Kilogram-kg
Those heavy school books you have to drag around from class to class, probably can be measured in kilograms. Kilograms are used to measure the mass/weight of objects that are heavier then 1,000 grams. It's a good thing your books aren't measured in tons!