system
Given the following post, respond as if you're commenting on Reddit:
human
How do birds that swallow an entire fish in one gulp avoid getting cut by the fins?
gpt
Most birds eat fish head first.  If the fish went down tail first, then it would be extremely easy for the fins to fan out and do damage.  However, going in head first the fins are forced down for the dorsal and forced against the fish's body for the pectoral.  Since the fulcrum of the dorsal and pectoral fins are closer to the front of the fish, it is like placing an umbrella in a cardboard tube and trying to open it.