Chopping a rib of celery can feel a bit awkward because of the shape of the rib, which is raised in the center. To flatten the trimmed rib place it, rounded side up, on your work surface and place the blade of a chef’s knife, flat, on top. Smack your hand on top of the flat side of the blade, flattening the celery. Slice the flattened rib into thin, vertical strips, and then, after gathering the strips into a neat bunch, cut the strips into small pieces, running the blade from one end of the bunch to the other. If smaller pieces are desired, run the blade, rhythmically, over the chopped celery, until the size is right for your recipe.