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A rubber duck with a vacuum inside (removing the air material) of it is just a piece of rubber with eyes. Assuming OP's point about the rubber not being less dense than water, it would sink, no?

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No. Air is less dense than water; vacuum is even less dense than air. A rubber duck will collapse if you seal it and try to pull a vacuum inside with air outside, but if the rubber duck is in a vacuum then it will have only vacuum inside and it will still float on a liquid the density of water. If you made a duck out of a metal shell you could pull a vacuum inside, like a thermos bottle, and it would float too.

The metal shell is ridged though, so the volume maintains the same with the vacuum. A rubber duck collapses with a vacuum inside of it, thus losing the shape of a duck and reducing the volume of the object =). That's why I said it's just a piece of rubber with eyes.

> A rubber duck collapses with a vacuum inside of it

Not if there is vacuum outside too. In a vacuum it remains a duck and still floats.

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