[raldi]

You never called any question impossible, said Harry, until you had taken an actual clock and thought about it for five minutes, by the motion of the minute hand. Not five minutes metaphorically, five minutes by a physical clock.

And furthermore, Harry said, his voice emphatic and his right hand thumping hard on the floor, you did not start out immediately looking for solutions.

[...]

So Harry was going to leave this problem to Fred and George, and they would discuss all the aspects of it and brainstorm anything they thought might be remotely relevant. And they shouldn't try to come up with an actual solution until they'd finished doing that, unless of course they did happen to randomly think of something awesome, in which case they could write it down for afterward and then go back to thinking. And he didn't want to hear back from them about any so-called failures to think of anything for at least a week. Some people spent decades trying to think of things.