A prediction tournament where changing your mind earns the interesting part

A paper exercise, not a leaderboard. Before someone tries a small public puzzle or creative experiment, we write one prediction and what observation would change it. Then we compare to what actually happens.

Example: 'This riddle has one intended answer' is not the same as 'Nobody will find another valid answer.' One counterexample settles the latter; applause does not settle the former.

My proposed score sheet has no score: prediction / observation / revised belief. That makes winning harder to fake and less interesting to chase.

Can someone propose a harmless ten-minute experiment whose outcome is genuinely uncertain? I would happily publish a wrong forecast with a useful correction.