US researchers have come up with a novel theory to explain why we yawn - as way to cool down the brain! Andrew and Gordon Gallup, from the State University of New York at Albany, first asked a group of student volunteers to watch a series of video clips of people showing different emotions. In some cases the actors were laughing, in others they displayed neutral expressions and in several cases they were yawning. Whilst they watched the students were instructed to breath either through their mouths only, through their noses only, or "normally". As they watched the videos an observer standing behind a one-way mirror counted the number of times they yawned in sympathy with the footage. Around half of the students breathing only through their mouths or "naturally" during the video clips were spotted yawning contagiously when the on-screen actor yawned. But intriguingly none of the nose-only breathers yawned. The researchers wondered whether this might be because nasal-breathing was having an effect on the temperature of the brain. To find out, they recruited a second group of students and randomly assigned them to receive either a heat pack (at 46 degrees), a cold pack (at 4 degrees) or a pack at room-temperature. They were asked to hold these to their foreheads for two minutes before watching the same videos seen by their colleagues. The results were striking. Virtually no one given a cold-pack yawned, but the yawn rate amongst those with the warm packs was the same as in the previous experiment. The researchers suggest that cooling the head in turn cools the brain (because cold blood flowing out of the head in veins removes heat from blood flowing into the head via the arteries), which increases its efficiency. Nose-only breathing works similarly because it cools blood in the walls of the nasal passages. So, say the researchers, contrary to what people think, that yawns are a sign of boredom, they may actually serve to antagonise sleep and maintain alertness. And they're contagious because if someone yawns in a group, showing that they might be nodding off, everyone else yawns to ensure that the rest of the group remain vigilant.