Exercise 1: A Confounding Variable for Ice Cream and Drowning — Possible Solution
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A genuinely plausible confounding variable here is hot weather (or,
more broadly, summer/warm season).

Hot weather independently increases ice cream sales - people buy more
ice cream when it's warm out, for reasons unrelated to swimming at all.
Hot weather also independently increases the number of people
swimming, and therefore the real opportunity for drowning incidents to
occur, again for reasons unrelated to ice cream. Because both ice cream
sales and drowning deaths rise and fall together with the same
underlying seasonal temperature pattern, they end up correlated with
each other even though neither one causes the other directly.

This fits the chapter's own third category exactly: a confounding
variable (hot weather) causes both A (ice cream sales) and B (drowning
deaths) independently, producing a real statistical correlation between
A and B with no direct causal link between them at all.

ANSWER: Hot weather is a plausible confounding variable - it
independently drives up both ice cream sales (people want a cold treat
when it's warm) and swimming activity (and therefore drowning risk),
producing a real correlation between the two without either one
actually causing the other.

WHY THIS WORKS AS AN ANSWER
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This identifies a genuinely plausible third variable that independently
explains both halves of the correlation, correctly applying the
chapter's own "confounding variable" category rather than the
coincidence or reverse-causation categories.