1. **State the problem:** Simplify the trigonometric expression $\sin x \cos^2 x - \sin x$. 2. **Factor the expression:** Notice that $\sin x$ is common in both terms, so factor it out: $$\sin x \cos^2 x - \sin x = \sin x(\cos^2 x - 1)$$ 3. **Use Pythagorean identity:** Recall the identity $\sin^2 x + \cos^2 x = 1$, so $\cos^2 x - 1 = -\sin^2 x$. 4. **Substitute identity:** $$\sin x(\cos^2 x - 1) = \sin x(-\sin^2 x) = -\sin^3 x$$ 5. **Final simplified form:** $$-\sin^3 x$$