Imagine a clear, flexible-plastic thimble moulded with a protruding point on the end - this protrusion is used to bind the device onto the fly at the eye before the rest of the fly is tied. The fly is then dressed - baitfish with long, flowing mobile wings for best effect - and afterwards the 'thimble' can be flipped back on itself to produce a moulded fly-head. No more messing around with epoxy, silicone and flexible cements, this head is produced with a flick of a finger, and allows eyes and colour to show through. It's neat, it's quick, it's clean, but that isn't the clever bit.

When retrieved, flies with moulded heads of silcone, epoxy, or indeed this plastic cup, possess an undulating action which predatory fish sometimes find irresistible, especially with a pull-pause retrieve. All well and good. However, if you flip the plastic cup forwards so it now envelopes the eye of the hook, just like a plastic veterinary Elizabethan collar goes round a dog's head, you are now in possesion of a very different animal. Having already attached a length of nylon to the hook's eye before flipping the cup forwards, Marc proceeded to cast and retrieve this fly through his water tank. It's consequent sinuous, swimming action was an eye-opener. The fly quivered, wiggled and pulsed rhythmically through the water reciprocating the exact swimming movements of a small fish. Astonishing.

As soon as I saw it I was thinking: trout and fry patterns; Damsel Nymphs. I caught Gary Coxon's eye. He looked enthralled, excited, determined and somewhat furtive.

"Imagine using it for pike!" I exclaimed.

"I'm thinking of bass", he replied, and then he suddenly snatched one of my samples and ran off laughing hysterically into the home-going throng. Such is the way of the desperate fly fisher on discovering a major step to perfect baitfish imitation.

The simple plastic device (centre) added to the front of a Fry pattern means either a clear, shaped head can be produced or when the cup is flicked forwards, (bottom right), it imparts a realistic swimming action to the fly.In my time as editor of this magazine, I have seen various attempts to add the third dimension - action - to flies. Clipped deer-hair and siliconed diving vanes; plastic lips on wiggling Damsel Nymphs; and Roman Moser's addition of a sequin to the front of his Quiver series of flies have all been tried, but none match the simplicity and effectiveness of the idea I saw demonstrated at this year's British Fly Fair.

Magic Heads are available in three sizes from Lakeland Fly Tying, Crystal River and Farlows. A pack of six costs £2.95.