the sense of smell works because scent is actually a particle that is
recognized by a receptor in your nose. Scents diffuse in gases very
rapidly because the molecules in gas are more spread apart than a
liquid or a solid. When someone is baking cookies in the kitchen, one
might smell it from 5-10 minutes when the odor diffuses out. Sharks
can sense blood in the water from 1-2 miles away. once the blood is in
the water, they quickly arrive there. Not only can they sense that
there is blood in the water almost instantly, they can also trace it
back to its origin. How does the shark do this? assuming the blood
does not make a significant wave or splash, how would even one
molecule of blood travel two miles within seconds? or if not, how DOES
the shark know?

sharks may be use other complementary inputs:

http://library.thinkquest.org/J001458/instincts.htm

See the answer to Q:480348