Once at my best female friend's house, she was preparing to have a shower and put clean underwear on the toilet seat. I said I needed to go to the bathroom first so she said sure, and when I was in the bathroom I masturbated and then ejaculated all my cum into her panties. After I was finished, I flushed the toilet to avoid suspicion, and then got out and let her have her shower. While I waited for her to finish, I was almost freaking out because she would probably notice my cum when she put on her panties. After a while she emerged and didn't reveal anything was wrong,so I still to this day don't know if she had noticed or not, but the thought of her spending all day with cum stained panties was a nice turn on.