...assuming the construction of the reals, which is anything but trivial or basic.
Let's call the number 0.999... "x" and call "1"...well, "1". Let's say I maintain that the two are not the same, and Revoltingest rightly says I don't know what I am talking about because they are. But, being the gracious debater he is, instead of just writing me off he asks me to show that the two numbers aren't equal. We both start by imagining the positions of these numbers on the real number line. For me, they are very close but "obviously" do not occupy the same place, while for Revoltingest, who's mathematical knowledge surpasses my own, the two numbers are really one number located at the same place in the number line. As I think there are two different numbers here, I maintain that there must be some "distance" between 0.999... and 1 on the number line. Rather than call me a fool, Revoltingest asks that I tell him how far apart the number 0.999... is from the number 1. Clearly, I'm about to win this debate. All I have to do is provide a tiny, tiny, tiny, tiny difference- if there is any difference between 0.999... and 1, then they are two different numbers.
That's when I slam into a brick wall. No matter how small an amount I choose, e.g., 0.000000000000000001, 0.0000000000000000000000000000000001, 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001, etc., it turns out that the difference between 0.999.... & 1 is smaller than this amount. In fact, there exists no number small enough that 1-0.999.... is equal to that number. The distance between 0.999... and 1 is smaller than every possible number. That's because the 0.999...=1.