A childhood belief about sex

My friend was more precocious than I was in his interest in sex (though not much better informed), and he told me that when men and women had sex, they used a rubber. He wasn't sure exactly what it was, but I assumed it was some kind of appliance without which sex could not be done, since I couldn't imagine how any body parts that we boys had could have any physical correspondence with what girls had. Obviously, then, the rubber was the thing that would accomplish the hookup required to make a baby.

So one day I was downstairs in my grandma's old farmhouse, and I saw a box of what I now know to be milking machine parts, the long black rubber things you slip over the cow's teats for milking. They looked uncomfortably stiff, but since they had an opening about the proportion of a penis, I figured that was the rubber my friend had told me about! I was always really curious about the things, but carefully avoided looking at them when grandma was around, and certainly didn't ask any questions about them. (I also wondered why grandpa and grandma needed a set of four of them.)