What is it?不能说What's it？
What is it?不能说What's it？，当补语是代词的时候，助动词不能用缩略形式。为什么？还有哪些情况也是这样？Who is she?也不能缩略为Who's she?吗？

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理论上What is it?可以缩写为 What's it? 你看What's this？听起来一点不怪。一般不这样说的原因估计是怕混淆缩写，is = 's，has = 's，容易搞混而已。

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You can say ' It's what ?'