Often, we take as a given that something is impossible. Maybe it is simply that we were not able to do it in the past and we have not recently thought about the possibilities. Once you think about it, it might be possible to do (or at least come close to doing) many seemingly ‘impossible' things.
Generate three to five ideas in every round of use; a dozen or more ideas after three rounds.
Be imaginative when identifying things thought to be impossible. Realize that because we believe some things are impossible, we simply stop thinking about them. But "thinking about the impossible" is exactly the point of this tool.
Be prepared for silence and blank stares back as initial reactions. Avoid the impulse to back away from the question or try to give your own examples. Just wait and trust that the silence is equally uncomfortable for the group. Eventually, someone will speak up, probably starting off with the phrase, "Well, one thing you could do might be... ." Others will then join in.
At the same time, it is possible that you might pose something that completely stumps the group. Don't be discouraged. Smile and simply move on to another provocation or another tool.
All methods of creative idea generation involve some combination of mental attention, escape and movement. "That's impossible" focuses attention on things currently thought to be impossible (and, therefore, not worth paying attention to normally), escapes the usual thought pattern of simply dismissing the suggestion, and asks for mental movement to come up with ways to do the seemingly impossible.