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I looked at what she'd placed in my hand. It looked like nothing more than a wide-tipped marker, one with some odd features added. There was a button right near the gripping point, and a small turnable knob on the butt end. I twisted it experimentally; the nib cycled through the colors of the rainbow; red, orange, yellow, green, blue, indigo, violet, back to red...

"It's a rainbow pen!" Pinkie pie exclaimed. "Twi and me invented it!"

"It was her idea," Twilight Sparkle admitted. "It's filled with a special mix made of rainbow juice. It'll write on anything--- even thin air."

I raised my eyebrows. "Just push on the button with your thumb to draw on the air!" Pinkie said, hopping up and down. I complied; sure enough, the rainbow pen left a thick, wavering line of vivid rainbow red in the air. I waved my hand, entranced; my fingers passed right through it. "If you wanna erase, just pull the knob out." I complied again, the tip turned gray. I ran it over the line and the pen seemed to.... there is no other word for it... slurp up the red rainbow line. "that way it saves ink!" Pinkie said, grinning.

"I looked all over for something like this," Twilight said. "Well, I noticed how you always seem to be talking with your hands, trying to sketch things out, and I thought... well...." she shrugged. "It was Pinkie Pie's idea to use rainbow juice for ink, I modified the pen so it would work like a miniature rainbow protruder and so you could choose the color...."

http://www.fimfiction.net/story/24890/14/the-audience/chapter-14