Mark pointed the device at Heidi and pushed the “Age Transformation” button.
As he pushed the button, Mark discovered the button slid two ways – up and down, presumably to change the target's age from younger to older. He picked a direction to slide the button, and continued to hold it pointed at Heidi.
Mark pushed down the Age Transformation button and watched in amazement as his thirteen-year old sister Heidi suddenly grew up in the blink of an eye. She gained a full six inches of height, and blossomed out in her chest, hips, and rear. Her clothes altered themselves radically -- she was now dressed in a pink and blue college sweatshirt, a pair of old blue jeans with holes in the knees, and a pair of dirty, white tube socks with no shoes on.
"Very funny, Mark!" she said, suddenly spying him. She held the rubber snake and tossed it playfully in his direction. "I used to play pranks like this on your sister Jennifer back in the day. Ah, I'm glad I came back from college for awhile. It'll be good to get my laundry done and finally have clean clothes, of course... but it's also fun seeing my little brother and sister again."
Mark nodded silently, awed by the power of this strange, futuristic device he now possessed. It had really worked! It changed Heidi into a college-age girl... and she had no memory of things ever being the way they were before. She now believed that she was the eldest sibling -- not the youngest!
"Now out of my way, little man," said Heidi, emerging from her room with a basket full of dirty clothes. "I got laundry that needs doing, and don't you have school or something?"
Smiling mischievously, Mark began to plot what ELSE he could do with this little technological toy.