It’s a familiar enough scenario. You’re sitting in your lounge room, not expecting guests, and the doorbell rings. You have no way of knowing at that moment whether your visitor is, say, a bike-borne evangelist eager to audition you for the tabernacle choir, or, to borrow from the late Douglas Adams, the Ravenous Bugblatter Beast of Traal looking for lunch.

The potentially fatal indecision thus engendered, you will be pleased to know, can be easily dismissed through the purchase via Amazon of a nifty device called a UFO-02 Detector, described as a ‘‘magnetometer interfaced with micro controller for detecting magnetic anomalies’’.

Never assume: It is well known, of course, that spacecraft, even ones cunningly cloaked while parked in the front garden, play havoc with the electromagnetic field.

It is well known, of course, that spacecraft, even ones cunningly cloaked while parked in the front garden, play havoc with the electromagnetic field in precisely the way that bicycles don’t. Thus, for a measly $US48.54 ($52) you can henceforth be always forewarned of alien incursion, allowing a timely exit through the back door before the anal probing starts.

There’s an important point here. Technology is value-neutral. It can be designed by clever people for other clever people to use to do clever things. Or it can be designed by fruit-cakes for use by wing-nuts to track down their own imaginations.