system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What’s happening when the body shudders?

Today I was on a packed train and I was sat down, someone walked past me and their bag touched the top of my head very lightly and it sent a shudder wave from my head and down my whole body. What is actually happening/what causes it?
gpt
Look up ASMR. Certain parts of the body are more sensitive due to the way nerves are bundled, and a light touch or brush is enough to trigger that sensation you experienced.

Edit: Tickling works more or less the same way, except it's usually unpleasant, whereas ASMR can be mildly pleasant for some.