In 'birds have hollow bones because they facilitate flight,' the effect (flight facilitation) existed after the cause (bone structure), making the explanatory arrow run backwards in time.

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Explanatory arrow(as used in describing the direction of explanation): A metaphorical direction showing what explains what. If we say 'X explains Y,' the arrow points from X to Y. The statement worries that the arrow points the wrong direction when we explain hollow bones by their result (flight).
Temporal order (or 'backwards in time')(as used in explaining why this explanation seems weird or problematic): The sequence in which events happen. Normally causes happen before effects—you strike a match (first), then it lights (second). If the arrow runs 'backwards in time,' the effect would somehow happen before the cause, which seems impossible.
causation(Lewis's counterfactual theory of causation): Event C causes event E if and only if there exists a chain C, D1, …, Dn, E such that each member (except C) is counterfactually dependent on the preceding event; causation is the ancestral of counterfactual dependence
effect(Correlate of 'cause' in the defined causal relation)

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