My mother always supplied a box of tissues on my bedside table. Not sure if she knew what I used them for.
Same here. From the time I was 13 or so, my mom always put a box of tissues on one of the shelves in my headboard. I was a late bloomer, and she knew it, because she was at all of my well-child exams at the pediatrician. I was clueless, so I always thought they were for my nose, because I always had bad allergies.
At my exam on my 16th birthday, while I was lying on the table, and the doctor had my undies down for the genital exam, the first time either of them had seen my pubes and adult-sized penis, he asked me if my penis had started ejaculating semen. I said yes. He then asked when it started, and I said a few months earlier while I was staying away from home at science camp. He looked at my mom, and she nodded yes, because back then, I usually just ejaculated in my undies, which since they were white briefs usually resulted in a stain she'd see in the laundry. Finally, he asked if I was awake or asleep when it happened, and I said awake, basically admitting to masturbation. I was starting to become aroused, so he pulled my undies back up, said I had finally caught up with the other boys in my class, and told me to hop off the table.
For the standing hernia check, he sat on his rolling stool and had me facing him, with my back to my mom. For the first time ever, when he pulled my undies down for the hernia check (which he previously always did with my side to my mom), my penis was fully erect, pointing straight up at the ceiling. He said nothing about it and just went ahead with the hernia check, then pulled the waistband of my briefs up over my erection. He then turned me around for the scoliosis check, facing my mom with my five-incher straining against the waistband. Since she never caught me masturbating or even erect while naked, that was the most she ever saw.
The very next day, I saw a spare unopened box of tissues on my bookshelf. I guess before that exam she attributed the stains to wet dreams, but once she knew I was masturbating, she wanted me to use tissues. She never said they were for my semen, but she did remind me never to flush more than one tissue at a time, or it would clog a pipe. I always put tissues I used on my nose in the bin, not flushing them, so I figured it out. Since I took the hint, always putting a tissue over the head of my penis when ejaculating, she never said anything else about it. But two years later as she was helping me move into my college dorm, she put a box of tissues next to my bed. I panicked a bit when I noticed it, because I was nervous my roommate would see it. But then I noticed he had a box next to his bed, too. I wondered whether his mom put his there, too.