a sharp object can be used for protection & a cactus spine is a kind of sharp object ->  a cactus spine can be used as protection  harming something has a negative impact on / effect on that something & eating something causes harm to that something ->  eating something has a negative impact on that something  eating something has a negative impact on that something & some animals eat cacti ->  animals have negative impact on cacti  animals have 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 cactus spine can be used as protection & a cactus must protect itself from animals -> a cactus spine is a part of a cactus for protecting the cactus from animals