You're in the grocery store and a trying to figure out what to buy for dinner when someone down the aisle gets a call on their cell phone. Your thoughts of dinner are overtaken by someone else's monologue: "I'm in the canned foods aisle. What are garbanzo beans?"

Why are other people's cell phone conversations so distracting? Sure, some can drive you nuts because of their inanity, but why is overhearing a cell phone conversation more annoying than overhearing a face-to-face conversation?

There's no walking away from overhearing a one- or two-sided conversation when you're on a train or bus or worse, a plane. But the added mental resources that trying to understand half a conversation demands make it more distracting — and annoying.

The answer, according to an ingenious new study is that one-sided conversations, such as those cell phone conversations we overhear regularly, are harder to ignore and are more distracting than conversations in which you hear both sides. Like some advertising jingles, they just won't let you be.

A group of students from the University of San Diego were told that they were participating in a study examining the relationship between the ability to unscramble anagrams and reading comprehension. Each was given a paper with 30 anagrams to solve. While they were working on the anagrams, they overheard a scripted conversation that went on in the background about a birthday party, buying furniture, or meeting a date at the mall.

Half of the time they overheard a full conversation between two people in the room. The rest of the time, it was one person on a cell phone reciting his or her half of the conversation.

Afterwards, the students were interviewed and asked a series of questions about how the conversation affected them. They also were tested on their memory of the overheard conversation.

Students who had heard the one-sided cell phone call found the background conversation much more distracting than those who heard the full conversation between two people. And they also remembered more words from the conversation than students who had overheard the entire conversation.

When hearing only half of a conversation, it takes more attention to follow where the conversation is headed. This also makes the conversation more distracting than a two-sided one, which is much easier to dismiss as background noise.

There are certainly other reasons that cell phone conversations are annoying. Cell phone use on trains and buses and in restaurants and parks blurs the distinction between public and private spaces. There's no walking away from an annoying one- or two-sided conversation when you're on a train or bus or worse, a plane. But the added mental resources that trying to understand half a conversation demands make it more distracting — and annoying.

One encouraging finding is that students who overheard the cell phone conversation did no worse on their anagram task than the students who overheard a two-sided conversation. So all those annoying cell phone conversations may not be distracting you quite as much as you think they are.

The study is published in PLos ONE and is freely available.