US Standard Mass (Weight)
Mass: how much matter is in an object.
We measure mass by weighing.
(See Weight and Mass to find why weight and mass are different.)
These are the most common measurements of mass:
 Ounces
 Pounds
 Tons
The Ounce
The smallest unit of mass is ounces (oz).
One ounce weighs about the same as a slice of bread. It is very light.
The abbreviation for ounce is "oz", from the 15th century Italian word "onza".
But be careful: "Ounce" and "Fluid Ounce" are different!
 Ounce is mass
 Fluid Ounce is volume
Here we talk about ounce as a mass.
When we get lots of ounces we get a new label:
The Pound
When we have 16 ounces, it can also be called a pound (lb).
1 pound = 16 ounces
The abbreviation is "lb", because it came from the Latin word "libra".
Typically, this is the unit that you use to measure your own weight.
Pounds are used to measure lots of things from people to food to animals.
However, if something is very heavy we use yet another label:
The Ton
2,000 pounds is also one ton.
1 ton = 2,000 pounds
That is really heavy! Trucks, ships and heavy equipment are measured using tons instead of pounds.
An elephant has a mass of about 7 tons.
And this car is about 2 tons.
But in some countries a "ton" can mean 2,240 pounds, so to be clear use "short" and "long" in front of "ton", like this:

Final thoughts about measuring mass:
1 pound = 16 ounces
1 ton (also called "short ton") = 2,000 pounds = 32,000 ounces
1 long ton = 2,240 pounds
Weight or Mass?
We have used the word "weight" only because that is what people commonly say.
But we really should say "Mass". See Weight or Mass to learn more.
More About Ounces
"Ounce" normally means "Avoirdupois Ounce", but there is also "Troy Ounce"
 The International avoirdupois ounce is 28.349523125 grams (normal meaning of "ounce")
 The International troy ounce is 31.1034768 grams
More Examples
An ounce of mass is:
 about 30 grams
 exactly 28.349523125 grams
A pound is:
 a bit less than half a kilogram
 exactly 453.59237 grams (so a pound has a metric definition)
A short ton is:
 about the weight of a small car
 exactly 907.18474 kilograms
A long ton is:
 very close to 1 tonne (1,000 kilograms)
 exactly 1016.0469088 kilograms