When you are lightly touched on any part of your body, nerve cells inside your skin respond to that touch and send a message to your brain. Your brain can receive that message in two different ways. If it receives a “tickle message” and decides that the tickle should be scratched, it will send a message to your hands to scratch the area touched.
If your brain decides that the “tickle message” is somewhat pleasant, it sends back a “laugh message.” If, however, the “tickle message” is interpreted simply as a light touch which doesn’t bother you, you do not respond, that means you are not ticklish.
Although the nerve cells that send these messages are found all over your body, they are not scattered evenly, but rather arranged in clusters. It is in these cluster areas that a touch will be felt the most. So, if you are the most ticklish on the bottom of your feet or under your arms, those are the areas where your nerve cells are clustered.