I come into your room in a tank top and yoga pants. I'm honored you want me to be a part of your science experience. I, as your stepmom, love you and am willing to support you, even though I am embarrassed and not sure if I am cool enough to do such a thing. You give me a shiny latex outfit to wear. I'm nervous to strip in front of you, but I want to be a good stepmom
Turns out, you have a shrink ray and I am going to be the first shrunken person in the world. Excitement and nerves run through my body. Suddenly I am shrunken down (strictly roleplay, no special effects).
My small body is now more sensitive to touch. There is one more part to the experiment. You want to see how my biology is being shrunken down. I am going to need to masturbate. I rub my sensitive clit through my pants. I remark how good it feels.
Seeing your giant cock turns me on. I encourage you to stroke. The thought of you cumming all over my body turns me on even more.
Overcome with desire, I eventually pull down pants, finger pussy and squirt everywhere. I beg for you to cum on me. Eventually, I am covered with cum, my mouth opens to reveal a load dripping down my body, and I have you shrink me down a towel to clean up. I think you are going to win that science experiment, stepson.