Since the digit to the right of the thousandths place is "five or greater", you have to round up.
Since the digit in the thousandths place is a 9, you have to round up to a zero, and carry the 1.
But since the digit in the hundredths place is also a 9, you have to round that up to a zero, too, and carry another 1.
And so forth.
Since the first place from which you will not get a "carry" is the ones place (which has a zero), you naturally end up with 0.9996 rounding to 1.000.
Good work!
Eliz.