The Scarecrow believes he lacks brains, the Tin Woodman believes he lacks heart (emotion), and the Cowardly Lion believes he lacks courage. All three reveal they have precisely the trait they are certain they lack. This shows that what we do is more important than what we say, or, as the cliche goes, that actions speak louder than words. This also shows how inaccurate self-perception can be and the way people can lack confidence and underrate themselves if they listen to negativity (such as the crow who puts down the Scarecrow). Further, these three creatures who join Dorothy exhibit an endearing humility in their lack of self awareness.
The Scarecrow reveals his intelligence when he devises a chariot that can be pulled by mice in order to get the large, sleeping Cowardly Lion out of the poppy field. Another intelligent idea he has is building a raft to get them across a river.
The Tin Woodman reveals his heart or ability to feel for others when he cries after accidentally killing a beetle...
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