To most of us, a dog showing its teeth is a pretty clear signal of threat. In 1872, Charles Darwin first suggested animals showed emotions similar to ours in their body language, and we might be able to use this to better understand their behaviour and motivations.
There is perhaps no better species for investigating this idea than the domestic dog. We live alongside them, yet they have teeth that can inflict significant damage on a human. So we ought to be invested in knowing when they are happy, neutral or angry, at the very least.
