Usability testing means watching real people attempt real tasks with a design and noting where they hesitate, get confused, or fail — it reveals problems that designers, who already know how their own interface works, often cannot see themselves. In the 'think-aloud' method, participants narrate their thoughts out loud while using the product ('I'm looking for a way to go back... I don't see one'), giving the observer insight into their reasoning, not just their clicks. Usability researcher Jakob Nielsen's widely cited research found that testing with as few as five users is often enough to uncover the large majority of a design's usability problems, which is why teams run small, frequent rounds of testing and revise the design between each round rather than waiting for one large study at the end.