a pebble is a kind of small rock
pebble is small_rock. pebble is small_somewhat_round_rock. pebble is portable_object. pebble is size_relative_oblate_spheroid_stone. pebble is small_stone. pebble is small_round_stone_easily_grasped_between_thumb_and_fingers_of_human_hand. pebble is stone. pebble is rock. pebble is rigid_portable_object.