> (a) How could you reliably track this if the user always keeps their Kindle on airplane mode? Spotify's solution is to require a device to sync at least once

> (a) How could you reliably track this if the user always keeps their Kindle on airplane mode? Spotify's solution is to require a device to sync at least once every 30 days or the offline content expires. > (b) How could you track this accurately if the user reads a few hundred pages in the subway, where there's no Internet service? With a log of events that syncs when they do reconnect. > (c) How could you distinguish that between someone who hopped around a book in that same time period? By measuring the duration spent on each page.