Have you ever noticed that certain areas of your body are more ticklish than others? If someone tickles your shoulder, you might not feel a thing, but have them tickle your armpits or the soles of your feet and you're sure to squirm. That's because these body parts are packed with nerve endings, the fibers of the nervous system that perceive outside information and send it up to the brain.
According to Sarah-Jayne Blakemore, a neuroscientist at University College London, different parts of the brain activate if someone else is tickling you than if you're tickling yourself. When someone else tickles you and you start to laugh—a phenomenon known as "gargalesis"—both the touch and emotion and reward centers of the brain are activated. Some scientists think that this laughter may be a "false alarm" response. Our brains detect the startling contact as a potential threat, and then we laugh to signal to others that there isn't any danger after all.