This puzzle, while answered correctly, I believe is explained wrong. The end of the explanation states, "This means the crate marked "apples" must be 'oranges' and the crate marked oranges' must be 'apples and oranges.'"
What it should state is this:
"This means that the crate marked "oranges", which we know CANNOT be oranges since all of the crates were labeled incorrectly, and for the prior reason cannot be apples either, is forced to be the crate that contains apples and oranges. Now, since you know two of the three crates, it is obvious that the crate labeled "apples" contains oranges."
That is the same explanation, just pumped up with more words, which are unnecessary.
Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment