A carpet has a pile direction. The pile does not sit upright but points a little in one direction, giving a slightly smoother feel when stroked with the pile rather than against it. When something like a rug is placed on a carpet and walked upon, the pile bends in that direction, carrying the rug with it. With the release of foot pressure there is no longer the same amount of traction between rug and carpet, so as the pile returns to an upright position it does not drag the rug back with it.Traditionally, carpets are laid with the pile towards the door. This is from the pre-vacuum cleaner days when carpets were brushed clean and it was easier to brush the dirt out of the door in the direction of the pile.

Max Lang, Northampton, UK

I too have various rugs which travel rapidly on a cut-pile carpet, eventually pushing up against the skirting board.

After a period of careful observation I noticed that the rug always travels in the direction that the pile leans in, and only when someone walks on it. When the foot is removed, the flattened pile returns to its rest position but, like a ratchet, does not move the rug back to its starting point.

The movement is noticeable on cut-pile carpets, but hardly at all on loop-pile carpets, suggesting the effect depends on the pile structure, the length and stiffness of the fibres, the pressure of the footstep and perhaps the structure of the underside of the rug. Incidentally, it is easier to drag a rug - with or without chair on top - across a carpet in the direction of the pile rather than against it. I have not calculated the forces involved but they must be fairly large to move an armchair.

I feel an Ig Nobel award approaching.

Donald Brown, Kellas, Angus, UK