The aliens want to steal your brain… I think that’s fairly well established by now, but how to recognise them is less certain. Douglas Adam fans would assert that it’s the white mice you should be fearful of, while the followers of Roddenberry are more concerned about the delicious young ladies of Sigma Draconis VI.
In either case, it’s best to be prepared for all eventualities, and what better way than to have a spare brain you can hand over when requested? With a bit of luck, the aliens will be content with that, and leave you to boldly go about your other business.
My friend Richard has already got his. Here’s how he did it.