A friend recently told me a near-horror story about an email he almost sent to an administrator at the City University of New York. As he was wrapping up his email draft, his phone rang. So while on the call, my friend typed his subject line, “Attn: CUNY admin.”

At least, that’s what he meant to type.

He was ready to hit “Send” when his call ended, and my friend luckily had a chance to give his draft a quick review. Turns out, he missed the Y key in CUNY and instead typed another letter. Look at Y’s next-door neighbors—and imagine the worst.

The lesson: writing is a single-task activity. Study after study has shown that the multi-tasking strategy is a myth. Our brains can be devoted to only so many mental tasks at once. Try singing a song while adding a couple of three-digit numbers in your head. Or better yet, notice how a man turns down the car stereo while he’s looking for an address.

When someone is multi-tasking, they’re likely doing many things badly at the same time. And because writing requires so much of your brain power, it’s safe to say that almost any distraction while you’re writing can derail the whole process. So when it’s time to write, write.

Oh, and one final tip: don’t ever hit “Send” on an email while you’re on the phone.