Can we please stop using this phrase? By definition, it is an impossible entity, for obvious reasons. Usually it's used in a context which requires an example of something that might exist, but warrants no belief. Using a theoretically impossible thing constructs a faulty analogy, and can only serve to confuse. Invisible unicorn would suffice.
Besides, it's incredibly trite. Use your imagination, folks.
Besides, it's incredibly trite. Use your imagination, folks.