Dogs don't fetch balls because they are round - and actually understand the word 'ball' differently than we do.
When a dog hears the word 'ball', it remembers the rubbery texture, not the round shape - and runs off in search of that.
The animals attach words to objects in a fundamentally different way to humans.
The discovery could help to train dogs for specific tasks such as search-and-rescue - and could even make training family pets easier.
Many pet owners marvel at their dog’s ability to fetch different objects such as toys on instruction, perceiving this as evidence that the dog ‘understands’ words.
But the tests, with a five-year-old border collie called Gable, show that dogs associate words with textures - NOT shapes.
Dr Emile van der Zee from the University of Lincoln’s School of Psychology said: “A number of recent studies have suggested that the domestic dog’s word comprehension is human-like.
"Arguments have been made to refute this claim but until now there has been no clear empirical evidence to resolve the debate.
"Our findings bring a fundamental new insight into this discussion and add to our understanding of the cognitive equipment necessary for true human word learning.”
Dr van der Zee and his colleagues worked with a five-year-old border collie called Gable who had shown remarkable abilities to learn new object words.
They devised four different challenges for Gable to determine the extent and nature of his word comprehension.
On a number of occasions a selection of ten different objects known to Gable were placed in an enclosure out of sight of Gable and the researchers, and he was then given a verbal instruction to fetch a particular object from the ten.
Initial tests confirmed that Gable could easily distinguish between toys he knew well.
It was when the researchers introduced new words and objects of varying shape, size and texture that Gable began to reveal that it was texture, not shape that dictated what he brought back.
He appeared to make distinctions based first on object size, then, when he had longer to become familiar with the new objects, on the basis of texture.
Object shape appeared to have no influence.
The researchers concluded that the mental lexicon – the long-term mental store containing sound-to-meaning mappings – appears to be fundamentally different in dogs and humans.
Dr van der Zee added: “This would suggest that an important factor in the natural structuring of the mental lexicon may be the way in which sensory information is organised in a particular species.
The human visual system is tuned to detect object shape for the purpose of object recognition. In our experiments we excluded Gable using scent cues.
It seems that his visual system and sensory cues linked to his mouth region are focused not on shape, but on size and texture.