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Cell["Calculate the Area of a Circle", "Subtitle",
 TextAlignment->Center,
 TextJustification->0],

Cell["\<\
First a simple function definition.  Note the []'s and the capitalized Pi.  \
What does := do?  (That means \"delayed evaluation.\"  More later!)  Why does \
radius on the left side have a \"_\" after it?  (It means that radius on the \
right side is a dummy variable.  The \"_\" defines a \"pattern.\"  More \
later!)\
\>", "Text"],

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Cell["Test it out for a case where we know the answer.", "Text"],

Cell[BoxData[
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Cell["\<\
But suppose we want the answer to be numerical (i.e., with some number of \
digits).  Here's one way:\
\>", "Text",
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Cell[BoxData[
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Cell["\<\
But what if we want \"n\" digits precision?  Here's a new version:\
\>", "Text"],

Cell[BoxData[
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Always check your definition with a \"?\" in front of the name:\
\>", "Text"],

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Cell[TextData[{
 "We see that BOTH definitions are active.  How does ",
 StyleBox["Mathematica",
  FontSlant->"Italic"],
 " know which one to use?  Answer: By the number of arguments given to the \
function!  Try it:"
}], "Text"],

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Cell["\<\
Finally, let's write a \"program\" that asks the user for the radius.  First \
the definition.  The \"Module\" ensures that \"radius\" and \"area\" are \
local (as opposed to global) variables.  Note the use of semicolons.  (More \
later!)\
\>", "Text",
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Cell["Now run it.", "Text"],

Cell[BoxData["myarea"], "Input"],

Cell["That's all for now!", "Text"]
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