Years ago, when my wife, April, and I were getting married, we decided to DIY our wedding invitations to cut down on cost. What I didn’t know at the time was that this mundane task of assembling, addressing, and sealing 200+ invitations would teach me the most powerful productivity hack I use in business.

To begin, allow me to take you back to our pre-wedding, invitation-assembly bliss.

When everything was designed and printed, April and I sat there looking at a huge pile of invitations, RSVP inserts, envelopes, stamps, ribbon . . . the list goes on. I watched as April dove in and went through the full 2-minute process of completing an entire invitation before stopping her.

“Wouldn’t it be much more efficient to do one task at a time in totality?” I argued that my way of doing things would be much more efficient: first put the ribbons on all the invitations, then stuff all the invitations into envelopes, then stamp all the invitations, and so on. She dug her heels in and maintained her way was better. Of course, a friendly competition ensued, and we decided to split up the invitations 50/50 to find out who finished their pile fastest.

Now, at first thought you may think I won with my well-thought-out “mass manufacturing” style process, right? Wrong. My wife easily won by completing one invitation at a time, and I was left to realize that once again, April is always right.

But beyond that very valuable relationship lesson, I also had another epiphany click into place. That is, the cardinal rule of productivity: work on one thing at a time until completion.

In work and life, there are a few reasons why you never want to use the “one stage at a time” process I went with when compiling the invitations. I’ll unpack the reasons why that is.

Switching from task to task wastes time

It wastes precious time without you even realizing it. Every time you transition from one task to the next, you’re wasting seconds. Now, I realize it’s only a matter of seconds on the micro-level, but in the grand scheme of things, when you’re looking at a larger period of time, those seconds can add up to hours of lost time.

As Psychology Today puts it, each task switch may waste only 1/10 of a second, but if you’re switching several times in one day, it can actually add up to a loss of 40% of your productivity. On the other side of things, sustained effort on one project until completion allows you to get into that flow state where you’re fully immersed in focus and the rest of the world falls away. Additionally, working to completion allows you to achieve “psychological closure,” a human need so deep that Harvard Business Review states if a person is interrupted midtask, their need for closure is so strong that it will spill over and make him or her more decisive in unrelated tasks.

You risk not seeing errors until it’s too late

In Eric Ries’s book, The Lean Startup, he talks about the power of small batches. When things are done in small batches, it’s typically a more calming experience because you have more control over what you’re doing. You also engage in what Ries calls “validated learning.” That is, you learn what works or doesn’t work as you go and are able to readjust along the way if needed.

To use the wedding invitation theme of this article, let’s say April and I bought envelopes that didn’t fit our invitations. By doing one invitation at a time, April would find that out a heck of a lot faster than me, who wouldn’t be clued into this error until the very last stage of my phased approach. In achieving that validated learning, April would have run out to buy corrected envelopes long before I even came to the realization, making her more efficient once again.

Momentum and motivation is stifled

Funny story: When I was in high school, I worked at a Halloween costume factory. Each day when I clocked in, I was given my “job” for the 8-hour shift. These jobs were ridiculously minute: “fold the left sleeve over,” or “place each costume into a box.” For eight hours I would do that one job over and over again until I wanted to scream. Now, I’m not saying packaging a costume from start to finish would have been anything worth noting in my obituary. But at least I would have been able to count up how many costumes I successfully packaged in my shifts. It would have given me much more momentum throughout my shift, as well as motivation to come back the next day.

Which leads me to my last point. Using the “one thing at a time until completion” model will be more productive not only for you, but also for your entire team. Giving your employees a sense of psychological ownership of their jobs has been credited with dramatically improving their engagement, happiness, and productivity. Having this feeling of ownership also makes employees more helpful and generous toward others on the team.

That being said, the next time you have a project that needs to be completed, consider assigning it to one employee who can tackle it from start to finish. This approach, versus giving three employees one stage of the process each to work on separately, is going to reward you in the end because the employee will be more motivated to produce excellent work.

So there you have it. Remember, working on one thing at a time until completion is going to save you time and money in the long run, so think about where you can implement this in both work and home life. The result will be more progress, which at the end of the day is what we’re all looking for.

Pat Flynn is a father, husband, and entrepreneur who lives and works in San Diego, California. He owns several successful online businesses and is a professional blogger, keynote speaker, Wall Street Journal best-selling author, and host of the Smart Passive Income and AskPat podcasts.