In English, we can express the idea that something is not negative, such as:
A: What's in the box?
B: Oh, nothing.
A: It's not nothing!
In English, the double negative (not and nothing) conveys the idea that we are negating the "noting" said by person B. In Spanish, though, double negatives don't work the same way. A word-for-word translation would result in:
A: No es nada!
Which means "It is nothing" not "It is not nothing."
The same phenomenon is found in other contexts, as well, any time we would intentionally use a double negative to indicate a positive (as opposed to when a double-negative is unintentionally used, as is also often done).
How can one express the double-negative meaning that is present in the English original?