You have read an article, and there is an update or correction.
Use a cookie to identify the visitor, and alert them while they are next visiting your website about articles they have read and have been updated so that they can see the correction.
You go to a store (a book store for instance), and you are interested in a particular subject. You find the shelf you’re looking for, but there are so many options, that unless you already have some knowledge, it’s hard to decide what books to buy and in what order to read them.
Make an appointment with a specialist and specify what you are interested in (for instance books about the history of science, or productivity, etc.) You’ll meet at an agreed upon time at a big store, and he will do a whole tour of the subject with you, and give you an independent advice on what to buy.
Even though you may follow many public figures online, you don’t always keep up with them daily or even weekly.
Enable public figures to allow their fans to follow them geographically, so when they visit a city their local fans get an alert.