When I was quite young, my parents had our house remodeled, enlarging two rooms and adding a bath. This was desperately needed, as our family had grown to seven. I had a younger brother, a twin sister, an older sister and a younger sister, all of us close in age, except for the younger, "accidental" brother, who came much later.
My sisters had lots of friends, who often slept over. As I approached puberty, I became very aware of girls, getting more and more excited at the idea of their being naked in the same house, and seeing them in their nightgowns, bra-less. I frequently masturbated in the old bathroom while girls bathed in the other, which was off the girls' room, knowing I was not two feet away from a naked girl, the tub being just on the other side of the wall. Finally, I got an idea-- one that paid dividends for years to come!
The new bathroom had been constructed adjacent to the existing bath, with the old window being left in place, so that the only thing between the inside of the window and the new bath was a single, half-inch sheet of wallboard. I wondered if I could put an unnoticeable pinhole through that wall (pinholes and marks were common in our house of five kids), and be able to see into the other bathroom. When no one was around, I pushed a tiny nail through the wall of the new bath to the window of the old. Then, looking through and not being able to see much, I gradually widened the hole on my side without affecting the pinhole on the other, until I could see perfectly! Nobody ever opened the curtain over that window, I reasoned, as there was nothing to see, and, as far as I know, no one ever noticed it.