You can tickle your friend, and your friend can tickle you.

But you can’t tickle yourself. It’s something most people realize when they’re kids.

But for years, scientists have been trying to figure out exactly why you can’t tickle yourself—and if there’s any way to trick your brain so you can. When someone else tickles you, it activates a part of your brain called the hypothalamus, which prepares you for pain. Some scientists think you laugh when you’re tickled because you’re signaling submission to someone who has you in a vulnerable position.

That might also explain why some people really hate being tickled, or react by thrashing. So it makes sense that you can’t tickle yourself, because you can’t surprise your own brain. When you start moving your fingers toward your palm, stomach, armpit, or wherever, your brain is one step ahead.

It knows where your fingers will land and that they won’t hurt you. But when someone else is tickling you, you can’t predict precisely when and where their tickle attack will strike. The unpredictability makes you more sensitive to the touch, and you react by laughing.

It also seems like a sense of agency is a key ingredient. The feeling that you are the one in control of your body’s movements means your own touches don’t feel like tickles. People who have schizophrenia, a neurological disorder that causes hallucinations, among other symptoms, seem to lack this awareness about their own movements.

And they can actually tickle themselves. That insight led researchers to try and come up with scenarios where people without schizophrenia could tickle themselves. In one small experiment, researchers tried waking people up during REM sleep, when they were probably dreaming.

They figured that since dreaming is similar to the hallucinations people with schizophrenia experience, the subjects might be able to tickle themselves if they tried right after they were woken up from a dream. But it didn’t work. Overall, waking people up from REM sleep didn’t give them the ability to tickle themselves.

In another experiment, researchers tried to make people feel like they were in somebody else’s body. They used a modified version of the rubber hand illusion — a psychological trick where you can get your brain to feel like a fake hand is your own if someone else touches it at the same time as your real hand, which is hidden from view. For this experiment, the researchers did a total body switch.

They had subjects look through video goggles that showed a feed from a camera on the experimenter’s head, and the experimenter sat across from them. Using the rubber hand illusion, they were able to make the subject feel like they were inside the experimenter’s body instead of their own. Then, while they were having this trippy out-of-body experience, the subjects tried to tickle themselves.

But even then they rated the sensation as being less tickly than when someone else tickled their palm. Their brains still knew they were in the driver’s seat, doing the tickling. So if you really want to be tickled, you’re gonna have to find someone else to do it for you.

And if you want to learn more about the human brain, check out our new channel, SciShow Psych, hosted by Brit and Hank, at youtube.com/scishowpsych.