Most of us grew up probably knew that you should not approach a dog if he snarls at you and cleaves his teeth – it’s a pretty clear body language hint not to approach. Although it is commonly confused with a snarl, a humble grin is not a signal to back off!
Submissive grinning, sometimes called smiling, occurs when a dog opens their front lips to expose their teeth (watch a video and a picture here) and often occurs during an initial greeting (dog to human, dog to dog). If you have never seen a dog do it before it can be quite confusing and it is often misunderstood by people because we are so conditioned to retreat when teeth are seen.
On the other hand, teeth bearing during resource guarding will often occur when the dog is in proximity to an item (bone, food, etc) and the dog is usually still and fanned over and shows teeth.
Not all dogs grin in a humorous way, but keep your eyes on those who do – it can be very fun!