At times, children play with the impossible.

Given the opportunity, about half of 18-to-30-month-old children will sometimes try mightily to climb into a toy car no bigger than a toaster, to sit in a dollhouse chair, or to glide down a teeny plastic slide, a new study of child behavior has found. This flair for treating small objects as if they're much larger betrays the toddlers' incomplete ability to integrate perceptions with appropriate actions, say psychologist Judy S. DeLoache of the University of Virginia in Charlottesville and her colleagues.

These so-called scale errors stem from immature connections between two visual systems in the brain's cortex, DeLoache and her colleagues propose in the May 14 Science. One system is devoted to perceiving objects in the world, and the other, to manipulating those things, the scientists say.

When a toddler treats a small toy as a large one, DeLoache theorizes, he or she accurately perceives the object's identity as, say, a chair or a car, and then acts on it in a familiar way without receiving a neural update on its diminutive size. Immaturity of another brain region considered crucial for assessing the appropriateness of planned actions also contributes to these blunders, she suspects.

"It's not clear whether all toddlers or only certain ones are capable of making scale errors," DeLoache says.

The researchers studied 29 girls and 25 boys from middle-class families. An experimenter observed each toddler in a playroom that included an indoor slide, a child-size chair, and a toy car that a child can sit in and propel by foot. After the children played with each large toy, researchers replaced those items with miniature replicas.

While playing with the small toys, 25 of the 54 children committed a total of 40 scale errors. Signs of serious intent typified such acts. For instance, one girl tried to squeeze a foot through the tiny car door, then removed her shoe and tried again.

Scale errors differed from pretend play, which often consisted of pushing the little car around on the floor and making car noises or sliding a doll down the small slide.

Observations of eight additional toddlers indicated that scale errors don't reflect a general difficulty in estimating sizes. When shown a large toy and its tiny replica and asked by an experimenter to perform an action--say, "Drive the car over here"--children always chose the large item.

Toddlers' scale errors are "fascinating, but it's not clear how to explain them," remarks neuroscientist Adele Diamond of the University of Massachusetts Medical Center in Waltham. Slipups in the budding ability to reason about objects may contribute to such behavior, she says.

The sight of an object automatically activates brain processes linked to using that object, notes psychologist Linda B. Smith of Indiana University in Bloomington. A momentary lapse of attention may keep a toddler from taking into account size information about an object, she says.