Describe the fallacy in the following "proof" by induction:
Theorem: Given any collection of
n blonde girls. If at least one of the girls has blue eyes, then all
n of them have blue eyes.
Proof: The statement is obviously true for
n = 1. The step from
k to
k+1 can be illustrated by going from
n = 3 to
n = 4. Assume, therefore, that the statement is true for
n = 3 and let
G1,
G2,
G3,
G4 be four blonde girls, at least one of which, say
G1, has blue eyes. Taking
G1,
G2, and
G3 together and using the fact that the statement is true when
n = 3, we find that
G2 and
G3 also have blue eyes. Repeating the process with
G1,
G2 and
G4, we find that
G4 has blue eyes. Thus all four have blue eyes. A similar argument allows us to make the step from
k to
k+1 in general.
Corollary: All blonde girls have blue eyes.
Proof: Since there exists at least one blonde girl with blue eyes, we can apply the foregoing result to the collection consisting of all blonde girls.
(Here comes the funny

)
Note: This example is from G. Pólya, who suggests that the reader may want to test the validity of the statement by experiment.