<p>That's a very normal part of cognitive development. Right now my twins call all fruit either "apple" or "orange" or "banana." They can say plum, pear, lemon, etc, but always return to these root concepts. They are just learning about categorization.</p>
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<p>Piaget wrote about it, here's a quote I found that explains his theory:</p>
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<div class="quote-container"><span>Quote:</span>
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<div> Schemas are continually being modified by two <a class="alnk" href="http://www.answers.com/topic/complementary" target="_blank"><span style="color:#0066CC;">complementary</span></a> processes that Piaget termed <a class="alnk" href="http://www.answers.com/topic/assimilation" target="_blank"><span style="color:#0066CC;">assimilation</span></a> and accommodation. Assimilation refers to the process of taking in new information by incorporating it into an existing schema. In other words, people <a class="alnk" href="http://www.answers.com/topic/assimilate" target="_blank"><span style="color:#0066CC;">assimilate</span></a> new experiences by relating them to things they already know. On the other hand, accommodation is what happens when the schema itself changes to accommodate new knowledge. According to Piaget, cognitive development involves an ongoing attempt to achieve a balance between assimilation and accommodation that he termed <a class="alnk" href="http://www.answers.com/topic/equilibrate" target="_blank"><span style="color:#0066CC;">equilibration</span></a>.</div>
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<div>(from <a href="http://www.answers.com/topic/cognitive-development" target="_blank">http://www.answers.com/topic/cognitive-development</a>)</div>
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<p>That means your daughter has a strong association of "cat" meaning "cute, small, fuzzy four-legged animal." All animals that fit that description are <em>assimilated</em> into this meaning of "cat." So rabbits, squirrels, foxes, etc, may all be called "cat." As she refines her definition of cat to be more specific, then she will start saying "rabbit" and "squirrel" etc more consistently. But first she has to lear how to <em>accomodate</em> her definition of cat. </p>