The Whispering Earring

The Whispering Earring is a device described in Scott Alexander's short story titled Clarity didn't work, trying mysterianism. One paragraph from the story describes it well:

The earring begins by only offering advice on major life decisions. However, as it gets to know a wearer, it becomes more gregarious, and will offer advice on everything from what time to go to sleep, to what to eat for breakfast. If you take its advice, you will find that breakfast food really hit the spot, that it was exactly what you wanted for breakfast that day even though you didn't know it yourself. The earring is never wrong.

As you can imagine, there is a steep price to pay for this perfect guidance. The original was posted on Scott Alexander's LiveJournal which is archived in many places.