"Once upon a time there lived a pink unicorn in a big mushroom house with three invisible potatoes. Could you finish the story for me in a creative way and explain why the unicorn ended up painting the potatoes pink?"

Well obviously, the unicorn did it to satisfy the ghost of Carl Sagan, who showed up at the unicorn's house and started insisting that the potatoes weren't there. Annoyed, she tried throwing flour on the potatoes to convince him, but it turned out the potatoes really were permeable to flour. It was touch and go for a while there, and even the unicorn started to doubt the existence of her invisible potatoes (to say nothing of her invisible garden and invisible scarecrow - but that at least had done an excellent job of keeping the invisible birds away). Eventually, though, it was found that pink paint coated the potatoes just fine, and so Carl happily went back to his post co-haunting the Pioneer 10 probe. The whole affair turned out to be a boon for the unicorn, as the pink paint put a stop to a previously unfalsifiable dragon, who had been eating her potatoes (or so she suspected - she had never been able to prove it). The dragon, for his part, simply went back to his old habit of terrorizing philosopher's thought experiments.