Why Is Grandpa On Top Of Grandma

I remember back when I was little, I would spend my summer vacations over at my grandparents house. I would watch my Grandma knit and listen to grandpa`s old war stories. I spent many good years with them. Everything was great until one year when I was older I stayed over for a couple of nights and everything changed. I remember it vividly, I was sleeping when I heard awful screeches and moans coming form my grandparents bedroom, so I ran in as fast as I could. After seeing what I saw that night I would never be the same. They were both naked going at it like jackrabbits. My grandma was sucking on my grandpa`s old dick and he stuck it in her over and over, it was so messy! They fucked all night long, and in every way possible. I was so confused I called my mom and asked her "Why is grandpa on top of grandma?"