CoachThe mass of a thing tells how much stuff it is made of. The more stuff, the greater the mass — and the harder the Earth pulls it down, which is why things with more mass feel heavier in your hand and press their pan of a balance scale down.
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Example 20.2 (Big is not heavy)
A pillow fills your whole arms, yet you carry it easily. A stone smaller than your fist strains your wrist. The pillow is big but holds little stuff — mostly feathers and air; the stone is small but packed with stuff. Mass is about how much stuff, not about how much room it takes. (These two ideas, and the tricky dance between them, meet again in the floating-and-sinking story one day.)
Example 20.5 (Other scales)
The kitchen scale and the bathroom scale have no second pan: you put the load on, and a needle turns (or a number lights up) to show the mass directly. Inside, a squeezed spring does the comparing for you. Handy and fast — but the old two-pan balance hides no machinery at all: just the lever rule you already understand.
Example 20.6 (Reading the market)
“A kilogram of apples” means: apples until the scale balances a block — about or apples. Flour comes in bags, chocolate bars are often , a letter needs about of paper, and the postman’s rule “under , one stamp” is a mass rule.