"Don't let the dog know you're afraid," most of us were warned as children. The idea was that if the dog knew we were afraid, it would take advantage of our fear by becoming more aggressive.
But is it true that dogs can sense fear?
The answer is yes, if you don't get too literal. As far as anyone knows, dogs don't have eerie supernatural powers. There's a scientific explanation for most of the amazing things dogs can sense. Often, that explanation has to do with the dog's nose, its No. 1 way of understanding the world. Dogs have many more scent receptors in their noses than humans do, and their receptors are sensitive to more smells. Some estimates are that dogs' sense of smell is millions of times keener than ours. [source: Horowitz]
So, a dog may not read your mind and think fear, but it can smell what happens in your body when you're afraid. Humans sweat when we're stressed. And if we're frightened, our bodies release adrenaline. Our blood starts pumping faster, and we emit pheromones, or chemical molecules that float through the air. Dogs can smell all these fear-triggered human responses.
Dogs' eyes are a secondary sense for them. Because they don't depend upon their eyes for their primary understanding of the world, they pay more attention to individual details than humans do, rather than processing an entire scene. That means that they detect small signals — facial expressions, tensing, varied gait — that tell them we are afraid.
If you're afraid, a dog will know through smells and visual hints. Whether that means the dog will become aggressive depends upon the dog and the situation.