It’s a doghouse on a stick, and it’s filled with books. At least that’s the simplest way I can think of to describe a Little Free Library. Maybe you’ve seen one; there are currently over 20 of these unstaffed, micro-libraries in Baltimore, according to the Sun. And their popularity is growing.

The idea is almost too simple too explain. Take one of the books if you want. Then return it once you’ve read it. Or don’t, no one will stop you. But you’ll probably feel guilty if the collection becomes depleted and end up bringing a couple books from home to stock it with.

The project began with Todd Bol in Hudson, Wis., in 2009. As a tribute to his former school teacher mother, Bol constructed a miniature one-room schoolhouse on a post in his front yard and filled it with books. It was a hit with his neighbors. Then Rick Brooks came along, and the two launched the project as a worldwide social enterprise.

Now there are Little Free Libraries on all the habitable continents. While, they’re no substitute for a conventional library (what are the odds you could find a specific title you needed among 15-or-so random books?), witnessing a social venture with no profit motive and run entirely on the honor system is good for the soul.

For more on Little Free Libraries in Baltimore — and how to start your own — read the article in the Sun.