Why can't we resist...?

Why can't we resist touching things?

The crisp fold of concrete, like a freshly starched and ironed shirt is a signature of I.M> Pei's National Gallery East Wing facade in Washington DC.

This design element is so remarkable, that passersby are drawn to the knife-like edge and as if drawn by a siren-song, are lured to touch the edge.

In fact this edge has been touched so many times since its erection in 1978 that the lower part of this crisp edge has become eroded, burnished and stained by the oils from the hands rubbing along the surface (see the lower part of the darker brick wall on the right, adjacent to the ground hedges).