Space and birthdays.
@5point9billion is a Twitter bot created by Matt Webb.
Light left the earth on the day you were born…this bot tells you when it reaches the stars. You follow it, tweet it the date of your birth, and then it lets you know whenever you reach Tau Ceti (or wherever). It’s a long experience, one that happens over several months and years.
Read more about the bot on the author’s blog.