On which interval is $f(x) = x^2$ strictly decreasing?

Explanation

1. 01

   For $x_1 < x_2 < 0$, squaring reverses the order: $(-3)^2 > (-2)^2$, for example. So $x^2$ falls as $x$ rises towards 0.

2. 02

   For positive $x$ it rises, so it is strictly decreasing on $(-\infty, 0)$ only.