A boy was walking along the street, holding his mother's hand like she had asked, when he spied a building made of glass. He let go of her hand and put both of his on the glass. It was an absolute no-no, but he had a good reason for it. He saw another little boy who looked exactly like him on the other side of the glass.
It wasn't a reflection. When he jumped up and down and waved his hands and yelled and pounded, the other boy merely watched. His mother by now had reached down to hold his hand again, and he asked her, "Mom, don't you think that boy looks like me?" The mother looked to where he was pointing and said, "Whatever do you mean, child? That's only a blank poster."
The boy looked back at the building and saw that his mother was right. There was a poster on the glass building, and when he went to either side of the poster, he didn't see anyone inside. But when he again resumed his position in front of it, there was the other boy looking at him from the other side.
His mother was tugging at his hand, tired of the game. "Come along, son," she said. "We've got things to do." So the boy grasped her hand and continued down the street. But when he looked back, he saw the little boy in the poster, the one who looked just like him, standing against the window now, waving goodbye.