A lazy eye is one that is healthy but does not see as well as it should, even with spectacles or contact lenses. It is caused when the eye does not have a clear image reaching the back of the eye when the patient is young (under 7 years of age). This can happen if the child has a cataract or a squint, or simply if they need spectacles but don't wear them. The degree of laziness can be reduced if the child has corrective treatment (such as wearing spectacles or patching the good eye to force the lazy one to work) when they are young. Children do not tend to notice if they have one eye that does not see as well as the other one, and if they do notice it they don't always realise that it is not normal (after all most people can write better with one hand than with the other, so why shouldn't a child expect that most people can see better with one eye rather than the other). This is one reason that all children, particularly those with a family history of similar eye problems in childhood, should have their eyes examined before they start school.