Today is a bit boring, let's guess a brain teaser.

There's only a half chance of guessing.

The swindler doesn't know in advance whether the stock will go up or down, but sends a text message to a group of people. Half of them will go up and half will go down, so no matter whether the stock goes up or down, half of them will always receive his correct forecast text message. Once the guessed thing happens several times, the deceived person thinks that the person is really a "prophet".