Teasing with her butt.

Her small round butt is too inviting for you to resist.

Still sitting down, you call out to her, invoking the magic word in your mind.

"Hey, you can sit on me."

She turns around and looks at you questioningly, saying in a lovely voice, "Is that fine? I don't want to trouble you too much."

"It's alright," you say. "I don't mind. Just treat it as a normal seat."

The beauty shrugs and agrees without hesitation, walking over to sit on your lap. She delicately lifts her skirt as she bends down. Her butt lands gently on you as she sighs in satisfaction, wiggling her butt to get comfortable, her slender body resting against you as she smiles cutely. Unbeknownst to her, the adjusting she did caused your previously minor erection to grow into a full-on hard-on. The knowledge that your erection was rubbing against her panties (which you had not even seen yet) made you nearly pant with desire. You sit back and enjoy the feeling of her panties on your bare erection, suppressing the urge to groan from sheer desire.

Wait.

*Bare* erection?

You gasp lightly as you remember something. Today you had put on a pair of pants with a zipper in the middle, and had forgotten to zip it up in your haste to test out your magic word. Couple that with the fact that you always go commando, and your erection easily slipped out of the zipper upon becoming hard. You shuddered with lust upon realizing all this, your sudden movement causing the teen to look at you over her shoulder with concern.

"Are you alright?"
"I'm good," you smile. "Very good", you thought.

"I have a request of you, though." You tell her. "I want you to tease my dick with your ass, and ignore everything I do unless I tell you otherwise. Got it?"
She nods and smiles understandingly, completely accepting of whatever you say, giving an "Okay" before turning around again to settle onto your lap once more.

You lean back and close your eyes, relaxing and enjoying the moment, as the beautiful teen sitting on your lap started moving.

Her cute butt slowly began to slide forward, your erection rubbing against her cheeks. She then began to slide her butt backward, and forward, and repeating the cycle at a slightly faster pace.

Her panty-covered cheeks hugged your erection, squeezing it as she began to move her butt left and right as well. She grinded her butt hard on your lap as you reached out and put your hands on her slim waist. You feel her panties surprisingly becoming slightly wet on your erection, making you groan with lust. Her amazing buttcheeks massaged your entire girth, beginning to move up and down, pleasuring your entire erection, the wetness beginning to soak through her soft panties.

She moaned quietly. It was barely noticeable above the bus, but you hear her gentle moan, and tighten your grip on her waist. Her butt is moving in a frenzy now, your erection being caressed by her panties, her buttcheeks, the wetness soaking through, her hair swaying, her head leaning on you with her eyes closed and her mouth barely open. She's enjoying this, and so are you. The pleasure is becoming intense, you're both breathing heavily, and you can sense your finish coming not too far away.