For most of human history, we've been fascinated with the idea of mind readers, psychics, oracles, and other "gifted" individuals who seemingly have an ability to know the future or read the thoughts of other people. It's not hard to understand this fascination, who wouldn't want to know the future, right? As for mind reading, we are all born into our own unique consciousness, and we live and die only ever knowing our own thoughts. The temptation to look inside the mind of another person is naturally pretty strong.
We associate those who can read minds with having some sort of supernatural ability, and many clever "magicians" have made fortunes out of their ability to convince people that their powers are real. In most cases, however, the reader simply relies on an ability to be intentionally vague so that you could interpret your own meaning in whatever they say. Readers also use cold reading techniques to quickly discern a few details about the client, and will often adjust what they say during a reading to match your reactions.
This video claims to be able to read minds, and gives viewers a simple set of instructions to follow. At the end, there's a good chance that most people got the number 3 and the country of Denmark (I got 3, but my country was the Dominican Republic). It might seem somewhat "magical" at first, but really this trick relies on two things: a natural tendency of most people to think of Denmark when asked to name a country starting with D, and a series of mathematical operations that will usually have the same end result despite what number you start with.