What measurement is
What does it actually mean to measure something, and how much can you trust the number you get?
Where you are 14 inside
What this area is about
To measure is to pick a unit and count how many copies of it fit. Before that comes deciding what you are measuring and comparing without numbers. After it comes a harder truth: no tool gives the exact amount, so every measurement is approximate.
How these fit together
Comparing without numbers. Attributes you can measure names length, mass and capacity as separate things. Comparing two objects lines them up; comparing with a middle object and balancing to compare mass extend it.
Units. What a unit is and iterating one unit turn comparing into counting. Smaller units give bigger counts. Reading a scale between marks and benchmarks help read a tool and choose a unit.
How exact. Precision of a tool, measurement error and significant figures say how far off a reading can be and how many digits to keep.
Using measurements. Story problems with grams and liters and word problems with mixed units put measured amounts into arithmetic.
A path through it
Attributes, direct comparison, then the middle object and the balance. Then what a unit is, then iterating. After iterating: unit size and count, and precision, then error, then significant figures. Reading a scale follows what a unit is; benchmarks follow measuring length. The story problems wait for metric mass and liquid units; the word problems wait for unit conversions.
Next: area, the protractor and the thermometer reuse the same unit idea.