When a user lands on a site, they have certain expectations for where elements like the search function might be located, what steps they might need to follow to accomplish their goal, or what would happen if they clicked on something.
We refer to these expectations as their mental model.
A mental model is what the user believes about a system, such as your website.
This model isn't based on facts, but is instead constructed primarily on the person's past experiences and what they *think* they know about the system.
These beliefs affect the way we need to design because people base their actions according to their mental model.
So when the system doesn't work the way people expect, they get confused.
For example, a really common mismatch I've seen in usability testing revolves around people's mental model for the browser back button.
People believe that the back button will always take them just one step back, to the previous state of the system.
That's their mental model.
Unfortunately, this doesn't always work as they expect.
