Mass and weight
When you ask how heavy something is, are you asking how much stuff it holds or how hard gravity pulls on it?
Where you are 3 inside
What this area is about
“How heavy” hides two questions. Mass is how much matter an object holds, and it stays the same wherever the object goes. Weight is how hard gravity pulls on that matter, and it changes from Earth to the Moon. On Earth the two move together, so everyday speech uses one word for both. This area gives each its units and then pulls them apart.
How these fit together
Units. Metric mass units (milligram, gram, kilogram, tonne) measure mass, with benchmarks to feel each one and a balance or scale to read it. Customary weight units (ounce, pound, ton) are the United States family, with 1 pound = 16 ounces and 1 ton = 2{,}000 pounds.
The distinction. Mass is not the same as weight separates the two ideas, and uses unit rates and the double number line to work out how much an object weighs under a different pull of gravity.
A path through it
The two unit lessons first. Neither needs the other: the metric one needs metric prefixes and benchmarks, the customary one needs the customary system. Mass versus weight comes later (grades 6 to 8), once unit rates are in hand.
Next: density, which divides a mass by a volume, and story problems with grams, kilograms and liters.