The contact lenses are very slightly weighted at the required lower edge such that when inserted into the eye they align themselves correctly.

This effect can also be caused by the "weighted" edge being slightly thicker and hence oriented downwards when the eyelid contacts it during blinking.

If you put them in upside down, you can always stand on your head.

Presumably astigmatics have to address this question also. Indeed anyone with a non-spherical-section cornea.

There are at least three kinds of bifocal contact lens: "translating", concentric and aspheric. The question relates mainly to the translating type, because these are the only ones that have to be positioned correctly all the time. A translating bifocal lens is in two halves, with the part that corrects for short-sightedness at the top and that for long-sightedness at the bottom, pretty much the same design as for bifocal glasses.

Such lenses are relatively small, covering only the centre part of the eye, and they are made of a more rigid material than the more usual soft lenses.

In addition, a small segment of the lens is cut away at the bottom leaving a flat edge. The flat edge ensures that the lens stays correctly aligned when you blink Its small size and rigidity allow the eyelid to hold the lens centrally while the eyeball translates - slithering up and down between far and near vision.

In concentric lenses, the centre is made up of concentric bands of material that alternate between correction for short sight and long sight. This design exploits the adaptability of the human brain, which can keep a mixture of both near and distant objects in focus at the same time.

The wearers "train" their brains to "see" either close-up objects or distant ones as required, because in effect the eye is registering both at the same time. Such lenses move with the eyeball and it doesn't matter if they rotate on the surface of the eye because they are symmetrical.

Finally, aspheric lenses are similar to the progressive lenses in glasses: correction for short-sight is in the centre of the lens, and it progressively changes to correction for long sight towards the outside. Once again the brain has to sort out conflicting information from the two parts of the lens.

People who wear progressive lenses will remember the first time they tried them, when head movements produced a wildly oscillating, completely disorientating visual field. Somehow the brain sorts this out and, after a few hours, everything returns to normal and perfect vision ensues.

Terence Hollingworth, Blagnac, France

I wear toric contact lenses, which correct for my astigmatism as well as my short-sightedness. Because these lenses are not symmetrical they have a heavier portion at the bottom designed to "drag" the lens into the correct position after insertion. The lens itself also has a small mark at the six o'clock position so I can insert it the correct way up when I start wearing them. This mark is invisible to the wearer once the lens is on the eye.

Rhian Chapman, Luton, Bedfordshire, UK