The friends were mountain biking in the Arizona desert and had been riding all day. While attempting to ride over some large rocks, one of the friends crashed and hurt his knee. In pain, he searched for a place to sit and assess the damage. He found a rock but it was actually a barrel cactus covered in needles. When he sat down, he jumped back up after feeling the needles in his rear end. He then spent a long time painfully removing the cactus needles.