Internal Structure and Density
A fluid takes the shape of whatever holds it. Liquids do. Gases do. Solids don't. Density, $\rho = m/V$, tells you how tightly mass is packed. The catch: density belongs to the substance, not the sample.
Three traps wait. First, bigger blocks are denser: no. A bigger block has more mass and more volume in the same ratio, and that ratio is density. Second, heavy things sink, light things float: a $5$ kg rock sinks, a $5$ kg log floats. What separates them is density, not mass. Third, density and mass are the same thing: no. Mass says how much; density says how tightly packed; weight ($mg$, in newtons) is a third quantity.
The work
3 ways in · any order
Lesson
Internal structure and density
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What makes something a fluid, density as $\rho = m/V$, and what "ideal fluid" means. Closes with a ten-scenario check (3 conceptual, 4 symbolic, 3 numeric) on the substance-vs-sample distinction.
Diagnostic
10-item topic check
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Ten algebra-based items on the common 8.1 mistakes: density treated as depending on sample size, mass/weight/density mix-ups, scalar slips, and diagram reading. Conceptual, symbolic, and numeric.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you've missed and grind it on its own. The round is adaptive: two correct in a row clears the misconception and you move on.