船一定要用有浮力的材料来做。
一种有浮力的装置，通常是圈状、带状或夹克衫状，用途是使人能浮在水中。
A buoyant device, usually in the shape of a ring, belt, or jacket, designed to keep a person afloat in the water.
不是所有有浮力的物体都能露在水面外，因为来自上面的水压随着水深而变化。
Not all buoyant objects ride the surface, as the water pressure from above varies with water depth.
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