Left Off

Recently I've been reserving the weekend to checking social media. This practice is balanced by reading a book throughout the week. It brought to my attention a contrast that posed a fascinating question:

When I return to reading a book, I have a sense of knowing where I left off. When I return to reading social media, do I know where I left off?

I'll check my Twitter feed every 10 minutes, but I scant remember what I read 10 minutes before. If I return to a book after a day, I have a better sense of where I left off.

I wonder why that is.