Try tickling your own side or the bottom of your foot. You know where your fingers are going, roughly how hard they will touch, and when the contact will happen. The sensation is usually much weaker than when somebody else does the same thing. So how does the brain know the difference between your own touch and someone else's?

Experimental research has found that self-produced touch is often perceived as less ticklish or less intense than comparable externally produced touch. One influential explanation involves prediction: when you make a movement, the brain has information that helps it anticipate some of the sensory consequences.

1. Your brain knows a movement is coming

Along with commands to the muscles, the nervous system can use information about the planned movement to predict what it is likely to feel like. If the incoming sensation matches that prediction, it may be given less sensory weight.

2. Surprise is part of what makes tickling powerful

When another person tickles you, you cannot perfectly predict the timing, direction, pressure, or exact path of every movement. That uncertainty helps explain the stronger response.

3. Changing the timing can change the sensation

Experiments that altered the relationship between a person's movement and the resulting touch found that making the touch less predictable could make it feel more externally generated and more ticklish.

4. This mechanism is useful far beyond tickling

The nervous system constantly distinguishes sensory consequences created by your own actions from events coming from the outside world. Otherwise every sensation caused by your own movement would compete for attention.

5. Can you trick your brain?

Under ordinary conditions it is difficult because you create the movement yourself. But if a device delays or transforms the resulting touch, the sensation can become more surprising.

6. Not everyone is equally ticklish

People vary greatly. Some react intensely, while others barely respond. Even the same person's reaction can change with mood, context, and who is doing the touching.

So the reason you struggle to tickle yourself is not merely that you consciously know it is your own hand. Your nervous system predicts the sensory result of your action and reduces its element of surprise.

Another person's touch is harder to predict—and that uncertainty is part of what makes it feel so different.