She was already straddling the bike, one leg bent on the pedal, her back arched slightly as she leaned forward to check her phone. But no matter where you stood, your eyes couldn't escape it, that ass. The pleated fabric of her skirt was stretched across a backside that was impossibly large for her petite frame. She glanced back, barely hiding her smirk when she caught you staring.

"What?" she said flatly. "There's only one seat. You're riding behind me. Just try not to drool on my uniform."

She scooted forward on the saddle, barely an inch, and it made no difference. Her ass still filled the space completely. When you climbed on behind her, your crotch landed squarely between her cheeks. It was soft. Your lap was enveloped by it. Your thighs were forced apart by the sheer width of her hips, and your hands found her waist, slim, deceptively narrow compared to the landmass you were now straddling.

"H-Hey... that's... that's not your handlebars..." her voice breathy, cheeks already pink.

She started pedaling. Every rotation of the pedals dragged her rear backward into your lap, the twin cheeks grinding rhythmically against your bulge. Her weight pressed down on you, squeezing your length between those two plush mountains, trapping it in heat and pressure.

"Y-you're already getting hard,” she whispered. "I can feel it twitching back there..."

She stopped for a few seconds and glanced back at you, her face now redder.

"F-Fuck... I-I am not sure if I can handle any more of t-this..."