The old biology textbook lay open on the kitchen table, its pages yellowed and filled with detailed diagrams of human anatomy. Alex and his sister Lena had discovered it while cleaning out their parents' attic on a lazy Saturday afternoon. At twenty-four and twenty-three respectively, they were both home from college for the summer, sharing the house while their folks were away. 'Look at this,' Lena said, pointing to a cross-section of reproductive organs. 'They make it sound so clinical. Like it's all just glands and hormones.' Alex leaned closer, his shoulder brushing hers. 'Yeah, but imagine actually testing the theories. The book says touch and sensation play a huge role in arousal.' Lena smirked, her green eyes sparkling with mischief. 'Witty as ever. So, Professor, care to demonstrate?' Their laughter filled the room, but the air thickened with an unspoken tension. They knew this was wrong—siblings exploring such things—but curiosity and a growing heat between them pushed the boundaries. Lena stood, her strong posture commanding the space. 'Let's see if the book holds up. No games, just science.' Alex swallowed hard, his pulse quickening as she began unbuttoning her shirt, revealing smooth skin beneath. The dialogue flowed sharp and teasing as they shed layers, hands exploring with deliberate slowness. 'Your skin's so warm,' Alex murmured. 'Does the book mention how a sister's touch feels electric?' Lena countered, her voice steady and confident. 'Better than any diagram. Feel this—my nipples are already hard from the idea.' They moved to the couch, bodies pressing close, fingers tracing curves and lines. The seduction built through witty banter: 'Think the book covers taste?' Lena asked, her lips grazing his neck. 'Only one way to find out.' Clothes dropped fully, leaving them naked and panting. Alex's cock grew hard under her gaze, while Lena's pussy grew wet and dripping with anticipation. She took control, pushing him back and straddling his thighs. 'No submission here—I'm leading this experiment.' Her hand wrapped around his hard cock, stroking with firm intent as she lowered her mouth for a slow, teasing blowjob, tongue exploring every inch. Alex groaned, sweating from the intensity. 'God, Lena, your mouth is incredible.' She pulled back, eyes fierce. 'Your turn to feel.' They shifted, her strong legs guiding him between her thighs. He tasted her dripping pussy, savoring the wetness as she moaned directives laced with wit. 'Deeper—science demands precision.' Tension peaked as he entered her, thrusting with building rhythm, her ass gripping his hands while she rode him dominantly. Panting and horny, they moved together, sweat-slicked bodies colliding until Lena came first, her walls clenching around his cock, followed by Alex spilling cum deep inside her. They collapsed, breathing heavily, the book forgotten on the floor. 'That was just chapter one of our research,' Lena whispered with a sly grin, hinting at more forbidden explorations ahead.
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