the.com/memento

A souvenir of a moment that refuses to become the past.

means An object you keep specifically to jog a memory — a physical reminder of a person, place, or event.

from Straight from Latin memento, meaning 'remember!' — the imperative form of meminisse, 'to remember.' It entered English the way a command sometimes softens into a noun: the word that once ordered you to recall became the thing that does the recalling for you.

etymology
Latin for remember, imperative mood, basically a command
film twist
Nolan told it backwards so you forget too
psychology
objects outlast memory, so we outsource remembering to things
paradox
the keepsake changes meaning as the memory it guards fades
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