Belling the cat
"Belling the cat" is an idiom which means to attempt, or agree to perform, an impossibly difficult task. It comes from the fable of the same name in which a group of mice debate plans to nullify the threat of a marauding cat. One of them proposes placing a bell around its neck, so that they are warned of its approach. The plan is applauded by the others, until one mouse asks who will volunteer to place the bell on the cat. All of them make excuses. The story is used to teach the wisdom of evaluating a plan not only on how desirable the outcome would be, but also on how it can be executed. It provides a moral lesson about the fundamental difference between ideas and their feasibility, and how this affects the value of a given plan.
- Belling the cat at Wikipedia