This happened with my sis when we were in our teens. My sis was the older of the two of us by about a year and a half.
We had the task of cleaning the leaves off the lawn one day. We raked the piles then bagged them. I held the bag while sis stuffed the leaves in. When she bent over to get leaves I noticed she had no bra on. Besides that, her top was a scoop-neck billowy blouse. Each time she bent I got a good long look at her tits... all the way to her belly button. And gorgeous tits they were. I had never seen tits before and couldn't believe my luck.
We worked for about ten minutes and she bent over dozens of times letting me get the peek of a lifetime on each bend (I now believe she was showing her stuff to me on purpose).
When we were about done bagging leaves I got the idea that she may let me go further somehow. I suggested we stop for awhile and asked her to follow me. She asked where and I responded, just follow me. I lead her behind a shed in the back of the lot. We lived in a small town and had a large lot. There were a lot of tall weeds there for perfect concealment.
When we got behind the shed I asked if she would bend front wards a bit for me. She obliged and I got another full view look. She looked me in the eyes and asked if I liked them. WOW!
She then pulled the blouse clear off over her head making her naked from the waist up. Beautiful, beautiful tits. Very firm, full, and pointy. I reached out and cupped my hands under them, then started to fondle them. I could tell by the way she responded that she loved my touch.
I reached for my pants zipper and zipped it down just to see what her reaction might be. Her hands went to her tits and I took that as a sign of approval. So I suggested maybe I could show her something. She said, 'go ahead.' So I pulled my now rock hard cock out of my pants and began to gently glide my hand up and down the shaft.
I had masturbated before so I knew what to do. Pre-cum was pouring out of the end and the foreskin was gliding over the slippery head very easily. I gently pumped my cock for awhile while she continued to fondle her tits. It didn't take many strokes before I shot a load of cum onto her belly.
I don't know if she has ever seen a guy masturbate before but I sure showed her how it's done. The ecstasy of the moment was breathtaking. We cleaned up the best we could and went back to finish bagging the leaves. I will never forget this occasion. We see each other often but never mention the incident. I often wonder if she remembers it with fondness like I do.