Next time you yawn in front of someone and they look at you as if you were bored of their company, just explain to them that your brain was feeling a little on the warm side, and yawning helps to cool it. While they might not believe you, you can always point them to an article on the Mercola website that explains some interesting results from a Princetown University study.
The researchers found that yawning performs an important function in cooling your brain.
The study showed that parakeets yawned more as their ambient temperature increased. The researcher argued that humans should actually yawn more in cold weather as that would be more effective in cooling the brain than yawning in warm weather (think of the temperature of the air you breathe in when yawning). This assumption was borne out in a study that showed people were more likely to yawn in Winter than summer.
You could therefore actually tell your friend that their conversation got you thinking so hard that your brain had started to over-heat, hence the yawn. 
Read the full article.