3girls, age_difference, ahoge, armpits, back, back-to-back, ball, bare_shoulders, beachball, bikini, blue_bikini, blue_bow, blue_sky, bow, breasts, brown_eyes, brown_hair, casual_one-piece_swimsuit, child, closed_eyes, closed_mouth, cloud, cloudy_sky, dappled_sunlight, day, head-mounted_display, holding, holding_ball, holding_beachball, innertube, last_order_(toaru_majutsu_no_index), legs, looking_at_viewer, medium_hair, misaka_imouto, misaka_mikoto, multiple_girls, navel, one-piece_swimsuit, open_mouth, outdoors, partially_submerged, pink_bow, polka_dot, polka_dot_bikini, short_hair, siblings, sisters, sky, small_breasts, smile, stomach, sun, sunlight, swimming, swimsuit, tautiki, teeth, toaru_kagaku_no_railgun, toaru_majutsu_no_index, upper_teeth_only, water, wide_shot