Something that was put out is pulled back in. The claws go back into the paw, the wheels fold back into the plane, and afterwards the surface is smooth again — as if nothing had been sticking out.
retraction 可以想成：伸出去的東西被收了回來。貓爪縮回肉墊裡，起落架收進機腹裡，收完之後表面平平的，看不出剛才有東西伸出來過。主體沒有變小，動的只是伸出去的那一截。英文把這個動作借去講話：說出口的話也像伸出去的手，收回來就是 retraction。
From the word’s history
re- 'back' + traction 'pulling' (or retract + -ion). The word literally names a pulling-back.
這個字可以拆成 re-（往回）加上 traction（拉），也可以看成 retract 加 -ion。字面就是「往回拉這件事」，所以實體的縮回是它最貼字面的用法。
Wiktionary entry for retraction (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1410580997:7110): 'From re- + traction or retract + -ion.'
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