Understand the reasons behind user behaviour, or uncover the mental models of the user in a usability test.
Ask users to think aloud (say everything they are thinking) while performing a task in a usability test. As thinking aloud is quite unnatural, you remind users regularly to keep on voicing his/her thoughts. Since children are bad at voicing their thoughts, this technique can only be applied from the early teens on.
The thinking aloud protocol is standard practice in usability testing, but can also be used in other situations to better understand the user's mental model.