ctic knowledge and conceptual knowledge and difficulties 
		with separating the essence from the accidental in our work with tools, 
		especially in the context of information-processing systems.</em></li>
		<li><em style="font-style: normal">The difference between performance 
		(what we do) and competence (what we understand about it) can be 
		immense.&nbsp; Deficiencies of our conceptual models and stories about 
		what the tool is are often not revealed until we make errors or there 
		are other breakd