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The natural self defense mechanism of the sharp cactus thorns can be used to depict many conceptual ideas.
abstract, ache, aggressive, arid, background, biology, cacti, cactus, caution, concept, defense, desert, dry, emotion, feeling, feisty, flora, heat, hook, hurt, keep, macro, nature, needles, off, pain, pierce, pierced, plant, prickly, razor, restricted, restriction, self, sensation, sharp, spike, spikes, spiky, spine, spiny, sting, texture, thorn, thorns, warning
Ouch. Great macro ;-)