Yesterday was the Little Girl’s last day in the “Purple Room,” which is what her nursery school calls the class for the school’s youngest children. She only went twice a week, for less than three hours, but the Purple Room was a very big part of her life.

There’s something so inexpressibly sweet about this age and this first experience of school. I’m having an emotion that I can only describe as preemptive nostalgia for this time. Her last morning there was yesterday, but already, I feel deeply sentimental about it.

The days are long, but the years are short.

For that reason, I’m so happy that I started keeping my one-sentence journal; otherwise I would worry that I wouldn’t remember any of the details about this time – the teeny tiny sinks, the coat hooks in the hallway marked with the children’s photos, the play kitchen and the board books.

Two years ago, I started keeping a one-sentence journal because I knew I would never be able to keep a proper journal with lengthy entries. I just don’t have the time or energy to write a long entry – even two or three times a week.

Instead, each day, I write one sentence (well, actually, I type on the computer) about what happened that day to me, the Big Man and the girls.

I can imagine one-sentence journal dedicated to more specific topics, as well. It might be useful to have one-sentence journal about your career – especially useful if you were starting a new business. It might be helpful to keep a one-sentence journal as you were going through a divorce, a cancer treatment, or other kind of catastrophic event. It would be lovely to keep a one-sentence journal when you were falling in love.

I posted about how one reader keeps a journal for his children.

I like keeping a one-sentence journal because it’s a manageable task, so it doesn’t make me feel burdened; it gives me a feeling of accomplishment and progress, the atmosphere of growth so important to happiness; it helps keep happy memories vivid (because I’m much more inclined to write about happy events than unhappy events), which boosts my happiness; and it gives me a reason to pause thinking lovingly about the members of my family.

One thing is true: we tend to overestimate what we can do in the short term, and underestimate what we can do in the long term, if we do a little bit at a time. Writing one sentence a day sounds fairly easy, and it is; at the end of the year, it adds up to a marvelous record.

From 2006 through 2014, as she wrote The Happiness Project and Happier at Home, Gretchen chronicled her thoughts, observations, and discoveries on The Happiness Project Blog.