You have 2 balls with identical diameter and weight. One is solid, one is hollow. How can you find the solid one?

Question

You have 2 balls with identical diameter and weight. One is solid, one is hollow. You can't tell by knocking on them. How can you find the solid one?

Answer 1

Drop it in water, which floats, is the hollow one.

Comments

If both are of same weight and diameter then either both will float or both will not float.
Same diameter means same volume.

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It doesn't say that material is same. So the weight can be same for same diameter.