When Ryder Carroll first created a video tutorial for the bullet journal, a system he created to stay organized, he only needed two tools: a pen and a notebook.

Now, exactly three years later, the wildly popular bullet journal has evolved beyond the pages of the simple Leuchtterum notebook that Carroll carries with him. If you browse Instagram using #bulletjournal or #bujo, you're just as likely to find bullet journals designed in calligraphy, colored pencils, and doodles as those that have been written just using a ballpoint pen.

Carroll says that for him, a minimalist layout is the most helpful aid for keeping track of tasks and staying focused. (He also makes uses of an index and various symbols to divide tasks up by type.) But he says he designed the bullet journal system so that it would be "flexible enough to handle whatever I threw at it"--and that others should be free to throw whatever they want at it as well.

"For some people, making the system significantly more elaborate motivates them to keep coming back to it--and that's what you want," Carroll says.

Yes, colorful bullet journals are more pleasing to the eye, but there's perhaps a broader explanation for why these elaborate journals have taken off. A number of studies have shown that doodling does wonders for your brain--it increases your attention span and you're more likely to remember something if you draw it out instead of writing it out.

Another possible explanation for the appeal? For some people, the bullet journal goes beyond a daily organizational tool and functions more as a journal. It contains their daily to-do lists, monthly budgets, and whimsical sketches--all within the confines of a single notebook. As such, it may inspire a more emotional attachment than a regular planner.

It's not uncommon to hear bullet journal bloggers say they love the system because they're able to fit "almost their entire lives" into their bullet journal. Heidi Currie, who runs the site The Bullet Journal Addict, says that she keeps a "monthly memories page" in her bullet journal where she will draw out a picture associated with an important event that happened in her life that month, such as the birth of her granddaughter or a recent vacation she took.

Jessica Chung, a bullet journal blogger who runs the site Pretty Prints & Paper, says that what appeals to her about the system is its "organized chaos." The only two things you have to include to officially make it a bullet journal: some type of bullet-like symbol to keep track of tasks, and an index to keep track of where you've put everything in the journal. So it's also possible that die-hard bullet journal fans feel a greater sense of autonomy in relation to their to-do lists--and a sense of autonomy has been shown to make some people more productive and more motivated.

"I am someone who likes pretty things, and I will do things if they're pretty. But what's really liberating for me about the bullet journal is that I'm able to design things myself," says Chung.

Want in on this trend? Before you start busting out the markers and drawing to-do lists in the forms of pyramids or circles, start small--especially if you aren't already in the practice of doodling daily. Carroll suggests starting with the basic bullet journaling system and then branching out and getting more elaborate once you get in the routine of bullet journaling daily.

Some super fans also agree that it doesn't always make sense to spend the time drawing out your to-do list. Both Currie and Chung say that works best for them is to keep two bullet journals--a more basic one for work tasks, when they don't have as much time to spend working on their to-do lists, and another, more elaborate one for personal use.