"There you are! What took you so long?"
My sister was leaning against the wall in the corridor, looking stressed. I guessed it had something to do with her project - the university professors apparently kept their master's degree students busy. Things were less hectic for us first-years - we could work really hard if we wanted to, but nobody was breathing down our necks like teachers did in high school, to make sure we did our homework. Having been at the university for about four months now, I had found a nice work rhythm that made sure I kept up, while still giving me time for hobbies and relaxation. I think my sister was working quite a lot harder than I was.
"Hey, sis. Got your message..."
"Yeah, that I sent two hours ago. And an hour ago. And half an hour ago. Where were you?"
"I had things to do - am I your servant now?"
"Yes! Well, at least... look, you agreed to help me with my lab project. You know the one that I'm right in the middle of?"
"That explains the ponytail."
My sister had long and wavy blonde hair that usually filled up a room when she let it loose. Wearing it in such a tight ponytail caused it to be pulled back, so she could keep it away from her face and put it down the inside of her clothes. She didn't usually wear it like this unless she really needed to keep her big mane in check, so I had guessed that she was working in the lab the second I saw her.
"You guessed I was in the lab because I have my hair like this? The lab coat didn't tip you off?"
"Oh... yeah..."
I actually hadn't noticed. People were wearing those white things all over the campus building, so I didn't really see a direct connection with lab work.
"Well, your hair looks great when you wear it loose, so..." I said, trying to get out of the doghouse with a bit of flattery. I knew she liked her hair. And it did look great.
"Don't change the subject. You agreed. Look, you're an adult now, you can't just make appointments and then blow them off. This project is important to me. I'll be presenting the results in class! On Monday! In front of the professors and everybody! I have to finish the lab work today if I'm going to have time to file the report before Friday. I have two hours of lab time left. After that, the microscopes are booked by the plant physiology course."
She looked at her watch and then back at me.
"Okay, I'm sorry. What can I do? You mentioned something about cell samples...?"
"Get in here."
She grabbed my shirt and pulled me in through a door nearby. Once inside, I realized it was a restroom.
"What are we doing in here?"
"I need a sperm sample."
"Uh... whaaa...?"
My sister produced a glass test tube from inside her coat. It was sealed with a plastic cap in order to prevent the contents from getting polluted. Right now, though, it was empty. Sis started talking quickly, clearly in a hurry.
"For the next stage in my project, I need to measure the motility and cell count of human spermatozoa under different conditions. I'm going to change variables like temperature, enzymes and pH in order to study the effects."
"B-but..." I stuttered nervously.
"Don't worry, you'll be anonymous. Nobody asks who the donors are anyway."
"But..."
"We often use male students for this, but there are only five guys taking the course this year, and they're rarely around, so I figured it'd be easier to just get you to do it."
"Yeah but... how do I..."
"Jesus, you really are clueless... You know how to make yourself cum, right? Just take this cap off and make sure you get it in the tube when you're ready to blow. Try not to spill any. The more I get, the better."
I looked around. It was pretty public, this place. It was a nice and clean restroom with several stalls, but even though it was empty at the moment, the corridor outside was full of faculty members and students walking by. I could actually hear the distant chatter through the door - talking, laughing, moving around... and there was no lock on the outer door.
"But ... can I do it ... here?"
"Oh, I'm sorry, not good enough for Your Highness? Well, I was gonna find somewhere a bit more private, and take my time briefing you about the procedure and everything, but you blew that plan out of the water by being two hours late! Now get in there and do it; you jerked off at least twice a day when we were living at home."
"You knew about that?"
"Hah. I could hear you through the wall. Look, it's easy, just think about some hot blonde, or that redhead you were staring at last week - she was pretty busty, right? Think about doing stuff with her. Dirty stuff. Mouth stuff. Butt stuff. Now go."
She turned me around and pushed me into a cubicle. The door closed behind me. I neglected to lock it, as I was still kind of shocked at this whole thing. I just stood there with the glass tube in my hand and tried to think straight. Sis was right, though, I had sort of blown her off when I saw the first message alert. Figured she could wait while I got some lunch. I had completely forgotten that I'd promised to help with her project, although I didn't know what the project was at that point. Still... there would probably have been more time to get comfortable if I hadn't wasted so much time. So yeah, she was now in a hurry and was probably justified in putting some of that blame on me.
I pulled down my pants. Tried to get comfortable. Tried to think about exciting things that would give me an erection. The pressure of the situation didn't help, though. The time pressure, my sister's annoyance with me, the public place we were in... I tried thinking about Beth, the beautiful redheaded girl in my botany class. She had freckles all of her face and a really nice rack that I always found it hard to keep my eyes off. I wondered what her boobs would feel like to touch. Imagined myself grabbing hold of them, touching and squeezing... I could feel my cock starting to respond. Then somebody came in.
I heard the sound of the outer door opening, and a woman's voice saying:
"Good afternoon, miss Johnson."
"Good afternoon, professor Williams." my sister replied.
Then I heard the sound of somebody getting in the cubicle next to me. It occurred to me that this was clearly the women's restroom. As if I hadn't had trouble getting hard already, now there was another thing serving to distract me. My dick started getting limp as I tried jerking it. Eventually, though, I knew I had to wait until the other lady had left the stall next to me.
Seconds ticked by. Possibly minutes - I didn't have a watch. It felt like minutes. At last, the lady flushed the toilet and left to wash her hands outside. I hoped nobody else would come in. If left undisturbed, I might be able to rub one out in a few minutes' time. As I waited, I could hear her exchange pleasantries with my sister out there. I almost blurted out:
"Oh, come on! Give me a break here, go to the cafeteria if you want a chat!"
Eventually, though, she left. Leaving me alone again, with my sister guarding the door to my stall.
"How's it going, bro? Done yet?"
"N-no... not exactly."
"What do you mean 'not exactly'? It's not rocket science or anything, just ... hurry it up!"
