The brain area that enables one to distinguish the different sounds made by a piano tends to be larger in a highly skilled musician than in someone who has rarely, if ever, played a musical instrument.
Evidence
The brain area that enables one to distinguish the different sounds made by a piano tends to be larger in a highly skilled musician than in someone who has rarely, if ever, played a musical instrument.
Conclusion
This shows that practicing on, and playing, a musical instrument actually alters brain structure.
Evaluate
This is a classic example of a Causal Flaw. As the legendary Steve Irwin would say, "she's a beauty!"
The evidence describes a correlation—two things that tend to happen together. A certain area of the brain tends to be larger in a highly skilled musician compared to someone who does not play a musical instrument.
The conclusion then claims that one of these things causes the other. While a correlation is potentially evidence of a causal relationship, by itself a correlation doesn't prove that one thing causes the other. This argument is flawed because it fails to consider alternative possibilities.
There could be any number of other factors which might cause this area of the brain to be larger in musicians. The actual causal relationship might be the reverse of what the conclusion claims: maybe when this area of the brain is larger than normal, that causes someone to become a skilled musician, or at least it's a partial cause.
Goal
We'll expect an answer to point out how the argument concludes that a casual relationship exists based solely on evidence of a correlation. A correct answer might suggest that the argument has failed to consider an alternative possibility. Beware of answers that describe flaws which don't occur in this argument.