Sedimentary clasts and grains become rounded primarily during transportation.

Here's why:

* Abrasion: As clasts are transported by wind, water, or ice, they collide with each other and with the surrounding environment. This constant impact chips away at the sharp edges and corners, gradually rounding them.

* Attrition: The process of clasts colliding with each other and breaking down into smaller, more rounded fragments is called attrition.

The longer the transportation distance and the more energetic the transporting medium, the more rounded the clasts will become.

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