In the example "Tom knows what Sue ate", let's assume that Sue had eaten a ham sandwich, and so, that means that Tom knows the answer to the question 'What did Sue eat?' which is that Tom knows that Sue had eaten a 'ham sandwich' -- But it does not mean that Tom personally knew that 'ham sandwich', not unless that ham had been Tom's pet pig earlier before it was slaughtered and cured and made ... More @Wikipedia
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