system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If smell is based off your nose reacting to particles, how do predators like sharks smell, for example, "a single drop of blood from over two miles away"?
gpt
They can't until some of the particles travel the 2 miles to reach them.  But they can detect very low concentrations of particles, so it doesn't take much for them to notice it, and then they can locate the source.