eating something has a negative impact on that something & animals eat cacti ->  animals have a negative impact on cacti  animals have a negative impact on cacti & if something has a negative impact on an organism then that organism must protect itself from that something ->  a cactus must protect itself from animals  a spine is a part of a cactus that is hard and located on the outside of the cactus & something hard / something strong can be used for protection ->  a spine is a part of a cactus used for protection  a cactus must protect itself from animals & a spine is a part of a cactus used for protection ->  a cactus spine is a part of a cactus for protecting the cactus from animals  a cactus spine is a part of a cactus for protecting the cactus from animals & protecting a living thing has a positive impact on that living thing 's survival -> a cactus spine has a positive impact on a cactus's survival by protecting the cactus from animals