Hello Anon, This is one I never thought about before, but "insert" would be used when the object being "inserted" will touch or come close to touching the sides of something, at least during some part of the insertion process. Any container that has a "neck" (like a bottle) will have a narrow entrance so even if the final placement isn't close to the sides, you have to insert it through the narrow opening, coming close to the sides. It would sound absurd to say "insert the pebble into the bucket," for example, because at no time is it necessary to pass the pebble close to the sides.
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