Categorization is crucial for successful living. Is this plant poisonous or edible? Who should I trust or avoid? Is this blob on the x-ray a tumor or normal tissue? Discovering what rules define category membership is called declarative category learning. Knowledge about rules can be easily transmitted to others verbally – lectures and books are ideal teaching tools for this, but this doesn't mean that language is required, since nonverbal animals can also discover rules and structures in their environment. However some degree of prefrontal cortex based working memory is most likely required for such abstractions. Some other categories may not be defined by an obvious verbal rule, and must be learned via trial and error, or learning by doing, which is called procedural learning. Learning complex skills, like riding a bike, or determining the sex of chicks are examples. Often these skills require integrating so many sensory elements, that keeping track of and combining all of them deliberately, exceeds the limits of working memory. Luckily, the neural plasticity machinery in the basal ganglia is well suited for converging multiple sensory inputs, and linking those inputs with rewarded responses. Interacting, but separate brain systems support these two modes of learning.