[...] By using stale metaphors, similes, and idioms, you save much mental effort, at the cost of leaving your meaning vague, not only for your reader but for yourself. This is the significance of mixed metaphors. The sole aim of a metaphor is to call up a visual image. When these images clash [...] it can be taken as certain that the writer is not seeing a mental image of the objects he is naming; in other words he is not really thinking. [...]