Psychologists are trying to unravel the reasons why children can draw crosses, but not an open pair of scissors.

Dr Robin Campbell of Stirling University says that finding out why children don't seem to be able to draw scissors could offer vital clues about the way they think.

By the age of four, most children can copy crosses, without any help or prompting.

And they are able to draw objects including houses, shapes, faces and people.

But they have trouble crossing the blades of scissors until they are at least a year older, with some six-year-olds still finding the task problematic.

Dr Campbell told BBC News Online: "Scissors really seem to catch them out, and we really don't know why that might be.

"Several plausible explanations have been put forward. It has been considered that children have trouble with what is called 'occlusion' - one object obscuring another.

"But many such explanations have now been eliminated."

Now psychologists are experimenting to see if children cannot draw scissors because they do not understand the relationship between the handles and the blades.

Dr Campbell said: "The drawings they do can be really wild. They are very charming pictures, and they will often get all the elements of the scissors there, right down to the pin holding the blades together.

Experiments

"But some drawings will show the scissors joined at the wrong end, and some will even show the elements entirely separate from each other - even the pin is floating in mid air.

"We are now going to look at the possibility that children can't draw them because they think the right handle operates the right blade, and the left handle the left blade."

Experiments will now centre on showing children colour-coded scissors, where the handle and its corresponding blade are painted the same colour.

Dr Campbell said: "It may be that this helps children to associate the opposite blade with the opposite handle and makes the object easier for them to draw."

The experiment will also feature scissors which are wrongly colour coded.

Vital skill

Dr Campbell said that the results of the experiment may help to demonstrate how important drawing is to a child's development.

He said: "In the 19th Century, drawing was a vital life skill. You could not enter a trade or profession if you could not draw, and so it was taught and encouraged in schools.

"What you will find is that from the age of about four, children love to draw and they do it spontaneously.

"But that interest peters out by the time they are seven or eight unless they are encouraged.

"I personally think that drawing is not just a reflection of intellect, but that it feeds intellect, and that in not encouraging children to draw, we may be taking something very important away from them."