You may not realize it, but every website you visit is created, literally, the moment you arrive. Each element of the page — the pictures, the ads, the text, the comments — live on computers in different places and are sent to your device when you request them.

That means that it’s easy for companies to create different web pages for different people. Sometimes that customization is helpful, such as when you see search results for restaurants near you. Sometimes it can be creepy, such as when ads follow you around from website to website. And sometimes customization can cost you money, research has shown. Orbitz showed higher-priced hotels to owners of Mac computers, for instance. Staples offered the same products at higher prices to people living in certain ZIP codes.

See the whole series here: ProPublica.org/blackbox