Every parent of a young child knows how emotionally attached children can become to a soft toy or blanket that they sleep with every night.
New research, published in the international journal Cognition, suggests that this might be because children think the toy or blanket has a unique property or 'essence'.
To support this theory, Professor Bruce Hood from the University of Bristol and his colleague Dr Paul Bloom of Yale University, USA, showed that 3-6 year-old children have a preference for their cherished items over apparently identical duplicates.
Children were introduced to a scientific looking machine that could copy any object but was in fact a conjurer's cabinet where an accomplice inserted replica items from behind a screen.
Professor Hood said: "When offered the choice of originals and copies, children showed no preference for duplicates of their toys unless the object to be copied was the special one that they took to bed every night. A quarter of children refused to have their favourite object copied at all, and most of those who were persuaded to put their toy in the copying machine wanted the original back."
It used to be thought that these attachment toys or transitional objects were comfort items that provided a sense of security for infants raised in households where they slept separately from the mother.
However, the results with the copy box studies suggest that in addition to these physical properties of the toy, children believe that there is some other property of their objects that cannot be physically copied.
This unique property also applied to objects belonging to famous people. Hood and Bloom placed a metal goblet in the copying machine and told 6-year-olds that the object was special either because it was made of a precious metal or because it once belonged to the Queen.