Density

Density measures how tightly matter is packed: the mass stuffed into each unit of volume. Two objects can be the same size with very different masses, or the same mass with very different sizes. Compute density by dividing mass by volume. On the metric scale, water's density is about 1 g/mL, meaning each milliliter of water holds about one gram of it.

Now the ship puzzle. Steel is about 8 g/mL, far heavier than water, so an ordinary steel knife sinks. But a ship is not a steel block; it is a hollow shell. Its total mass spreads over a huge volume because most of that volume is air. The ship's average density, steel plus air, works out to less than 1 g/mL, so the water can support it. Shape can beat substance.

This is why the float test is a density test, not a weight test. An object floats when its average density is less than the liquid's, and sinks when it is greater. Ice is the famous exception: it is about 0.92 g/mL, slightly less than liquid water, so ice floats even though most substances get denser when they freeze. Lakes freeze from the top down, which keeps life below the ice alive.

Sneak a check into every density calculation: the units write the formula. Mass in grams divided by volume in milliliters gives grams per milliliter, the same number as grams per cubic centimeter. Then compare your answer with water's 1 g/mL: rocks and metals land well above, oils and woods below. The comparison is the point of the number: a density of 3 g/mL means three times as dense as water.