What's the difference?

Question

When someone asks us: "What have you done?", how we should answer?
With present perfect? or we can use simple past?
For example" what have you done?" " I just opened the can" or 'I have just opened the can?' I think the second one is correct, but some say that both of them are correct. if so, can you explain the reason?
I don't want to know only which one is correct, but I want to know the grammatical reason. I am confused about the different tenses. The question is in present perfect tense but the answer is in the simple past tense? is that okay?the question and answer are correct? what's the explanation for this different tenses?

Answer 1

To me the second answer sounds much better. The question is "what have you done?" And it denotes a recent past. It would be preferable to give the answer as "I have just opened the can" which implies a recent past too. Had the question be " what did you do?" Then you might answer as "I opened the can". Again. The word just could mean two things. It could either mean you did the work very recently. Or, you have just done that particular deed and not anything else. So it might be better to be clear in the answer as to that.

Answer 2

Actually, you can use either the Past Simple or the Present Perfect and both will be correct. Take these example for instance:

• "Now, what have you done?"
• "I didn't do anything. It broke itself."

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• "What have you done today?"
• "Well, I washed the dished and I made a tasty cake"

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• "What have you done to him?"
• "I have already told you, I tried to reason him."

In your example both are correct. In speech I would prefer Past Simple in almost all cases. There's not much need for Present Perfect unless you really mean to use it. But I'm not a native speaker so that's how I see it.