01-intro.md: Improve the explanatory text in the "Variables as Sticky Notes" callout of Episode 1 (#928)

As discussed in #847, the example
in this callout makes it sound like the reason that variables don't
"remember" where their values come from is a consequence of the fact
that variables behave like sticky notes. However, this is inaccurate and
could mislead learners. For the moment, some modification of the
explanatory text has been suggested as a stop-gap until a better example
can be crafted.

Author: bielsnohr

_episodes/01-intro.md

@@ -136,14 +136,14 @@ weight in kilograms is now: 65.0
 
 > ## Variables as Sticky Notes
 >
-> A variable is analogous to a sticky note with a name written on it:
+> A variable in Python is analogous to a sticky note with a name written on it:
 > assigning a value to a variable is like putting that sticky note on a particular value.
 >
 > ![Value of 65.0 with weight_kg label stuck on it](../fig/python-sticky-note-variables-01.svg)
 >
-> This means that assigning a value to one variable does **not** change
-> values of other variables.
-> For example, let's store the subject's weight in pounds in its own variable:
+> Using this analogy, we can investigate how assigning a value to one variable
+> does **not** change values of other, seemingly related, variables.  For
+> example, let's store the subject's weight in pounds in its own variable:
 >
 > ~~~
 > # There are 2.2 pounds per kilogram
@@ -160,6 +160,11 @@ weight in kilograms is now: 65.0
 > ![Value of 65.0 with weight_kg label stuck on it, and value of 143.0 with weight_lb label
 stuck on it](../fig/python-sticky-note-variables-02.svg)
 >
+> Similar to above, the expression `2.2 * weight_kg` is evaluated to `143.0`,
+> and then this value is assigned to the variable `weight_lb` (i.e. the sticky
+> note `weight_lb` is placed on `143.0`). At this point, each variable is
+> "stuck" to completely distinct and unrelated values.
+>
 > Let's now change `weight_kg`:
 >
 > ~~~