CoachTo measure a length is to count how many times a chosen unit — a step, a hand span, a small stick — fits along it, end to end with no gaps and no overlaps. The answer is a number of units: “the room is steps long.”
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Example 7.4 (Try it: pace the room)
Cross your bedroom heel-to-toe, counting your foot-lengths. Ask a grown-up to do the same. You might count feet and the grown-up only — for the same room! Nobody counted wrong: your feet are shorter, so more of them fit. A measurement made with my foot cannot be checked with your foot.
Example 7.8 (Which unit for which job?)
Small things like a beetle, a stamp or your hand are measured in centimetres. Big things like a corridor, a bus or a swimming pool are measured in metres: the pool is long — imagine counting that in fingernail-widths! Choosing a sensible unit is half the work of measuring.