There was a college student (we call him "D") who loved pulling people's legs. For example, D would go to a restaurant with three friends, and when the waiter asked "four persons?" D would say "come on, look more carefully! There are five of us!"
D would do this often, and people began to think D could really see "it." But of course D couldn't see anything really; he was just amusing himself by watching people's reaction. No matter how many times his friends told him to stop it, all D would do was just laugh at them.
One day D, who was always with someone when he ate, thought he would eat alone just for a change, and went into a restaurant all by himself.
He sat down at a table, and a waiter brought him a glass of water and put it down in front of him... and in front of the seat opposite D's. "That's weird," D thought. Of course, there was no one apart from D at the table. "Maybe there was someone here just before I came in, and the waiter thought that the person was with me... Oh well, I will just move to another table if someone comes back."
The waiter came to take orders. D ordered a spaghetti dish.
By that time all other diners had left the restaurant, and D became the only diner there. It seemed now certain that the waiter made a mistake.
After a while the waiter brought him the dish; and to D's dismay, he again put the same dish in front of the opposite seat.
"Why do you put two dishes when there's only me here!?" D said to the waiter angrily.
The waiter, surprised, looked around him quickly. "That's strange! I'm sure I saw there were two of you."
Then D talked to the manager, who was at the door when D came into the restaurant, but he too said D was not alone but walked in with someone else.
D became very upset and stormed out of the restaurant.
Now being (he thought) at the receiving end of the joke he used to play himself, he realized how uncomfortable it felt. He swore he would never do it again.
But that was only the beginning. From then on, every time he went out people saw someone else beside D. Everywhere he went, whoever he met, D was seen with someone; even on a very crowded train people made a space, just enough for one person, only in front of D.
"Someone is following me...." D couldn't bear going out anymore. He grew paranoid, lost a lot of weight, and became a completely different person.
For many months he continued to live like a hermit.
He stayed in, just so that he wouldn't have to deal with "the other person."
Many days had passed. D gradually came to imagine that, after such a long time, "the other person" must have gone to somewhere else. He stepped outside for the first time in ages. The air outside felt fresh and good.
D went into a random restaurant. The restaurant had a lively atmosphere and D felt quite relaxed. However, he was still scared of sitting at a table so he chose to sit at the empty bar counter.
"I had been stupid," D thought. "There was nothing I should be scared of."
He couldn't have felt any better.
"Welcome," the waiter said, and put down a glass of water in front of D.
And he put another glass down next to D; and still another one next to it, and another one after that, and another one after that, and then another one after that...!