It's 6 a.m., and my phone is ringing.

I answer groggily, and a computer-generated voice offers me a job as a substitute teacher.

I jot down the grade level or subject, grab my "bag of tricks," rush to the school, and spend the next seven hours trying to control - and maybe even teach - a group of excited children.

I chase down kids who sneak out of the classroom; deal with kids who act out and then give a false name; and repeatedly line up 25 students to go to lunch, recess, and gym. These are students who know I'm "not their real teacher" and are determined to flout my authority.

At the end of the day I collapse in my favorite chair, feeling as if I've just run a marathon.

As a substitute I need to be ready for almost anything. If I show up expecting to teach English, I might instead teach Spanish or special education.

I might teach second grade one day and high-school French the next. I might arrive to find a lesson plan and a seating chart, or I might find neither.

I always carry a "bag of tricks" to help me - including name tags to stick on desks or shirts, generic lesson plans in case none were left, and word and number puzzle handouts for different ages in case of the dreaded "downtime."

It can be as frustrating as herding cats, as scary as bungee jumping, or as much fun as the best parts of being a parent - but it is never boring.

My first job was in a high school. I entered the classroom as a 21-year-old college graduate, and left it seven hours later a whimpering idiot.

They wouldn't sit down, some stood up and screamed, and the rest talked, laughed, or played radios. Raising my voice was pointless - I couldn't have been heard over the din without a fog horn.

I ended the day sobbing in the teachers' lounge. So why did I keep doing it?

First, because I didn't want to admit that these kids had gotten the best of me, and I saw it as a personal challenge.

Now that I'm a parent I do it because I know I'm as responsible as the schools are for my children's education, and it is my way of helping the schools succeed.

Besides, I wanted to see what my kids' school was really like when teachers and principals weren't on their best behavior for Back-to-School night.

Here's what I love about subbing: I love the young kids who are eager to learn, and I love helping the ones who are struggling to understand. I love telling stories to this wonderful, forgiving audience (they even laugh at my jokes).

I love teaching high school students, the ones on the cusp of young adulthood, who challenge what I say and by so doing teach me as much as I teach them.

And I love trying new ideas or recycling old ones. Some days I bring circles of yarn and teach cat's cradle while we're waiting for 3 p.m., and I have to push the kids out the door when the bell rings. Some days we sing, and I have them recite poems that I challenge them to memorize in one day.

Some days I bring games like "Napier's Bones," that teach multiplication skills.

In the end, subbing is my way of interacting with the future.

As John Whitehead writes, "Children are the messages we send to a time we will never see."

Being a "guest teacher" is how I choose to sign my own name on that communal message.

• Alison Hope Peña is a freelance writer and editor. She has taught as a substitute in six states for 30 years.