When we design, we make many assumptions whether we're aware of them or not. These assumptions can lead us to dismiss some solutions because we assume they're not possible. If we lay these assumptions out in the open, and challenge them one by one, we might come across solutions that we wouldn't have otherwise.
To use this method, list all of the assumptions you've made about the problem and its possible solutions. These are things that you believe are true and that you haven't questioned. Then, one by one, challenge that assumption by imagining that it is not true, and see what solutions would be possible if that were the case.
Even if the new ideas you come up with truly aren't possible, you might be able to use some aspect of them in your eventual solution.
Katherine is a designer who works on a mobile app. On one screen of the app, there's a table view that shows three columns. Katherine is asked to design an extra column in the table because new information needs to be added to each row. The problem is that the extra column won't easily fit in the width of the screen.
Katherine tries a few ideas like making the table scroll horizontally or shortening language where she can, but none of them are ideal. To come up with more ideas, she decides to list the assumptions she was making about the design. She quickly realises that she had assumed that the data needs to be in a table format. How might she solve this problem if it doesn't?
She designs a few new solutions, including an accordion pattern where the data can be expanded when needed, but otherwise each row is represented by a much simpler item in a list. She shows these to her manager, who likes the accordion idea and signs off on it.