How to explain common people what online tracking is?

Question

Every now and then, someone in my social circle wonders how they have been talking about some product or service with someone, only to see an ad for it a bit later in their social media feed.Then they wonder how this happened. Paranoia kicks in like "this phone is listening to us" or "they can read our encrypted chat messages".I've tried to find ways to explain this topic in simple, layman's terms, but mostly failed by going too much into details and confused them even more.What's equally important is that I'd like to give them actionable tips so they can protect themselves and their friends & family.What's your go-to explanation on this topic, and the best (simple) actionable advice on how they can protect themselves?

zelphirkalt · Accepted Answer

Maybe you can try an analogy with shop owners as servers. Then maybe the following works:
Tracking cookie - A visitor card, which will be forcefully checked at other shops, unless you throw it away.
Fingerpringting - The shop owner takes your fingerprint from anything you touched in the shop, or even from you paper money or pay card.
Tracking pixel - ???