There's an illusion that's become a kind of pet experiment that's used all over the world by psychologists who like spooking people.
Here's how it goes.
You sit at a table and hide one hand behind a divider with a blanket draped over your shoulder.
There's a fake hand on the table, and the experimenter arranges it so the fake hand's wrist looks like it's connected to you.
Then, you watch the fake hand as the experimenter strokes both your hidden real hand and the fake hand in exactly the same way.
After a few seconds, you start to get the sense that the fake hand is actually your own.
And that's when the experimenter grabs a hammer and whacks the fake hand with it, which makes you recoil in panic.
And yeah, it's kind of mean, but this illusion has actually turned out to be super useful.
It's helping programmers create better virtual realities, and it's also being used in medical treatments.
The trick is called the rubber hand illusion, and it was first published by a group of researchers back in 1998 when they discovered that if your real hand is hidden, you can convince your brain to adopt a fake hand as your own.
