system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do sharks sense a drop of blood in water from kilometers away?

A lot of documentaries say that sharks can sense a single drop of blood into the water from kilometers away. This way, they can find food easily. However, how is that possible? How can only a few drops of blood diffuse into the ocean and the sharks are still be able to sense everything?
gpt
Shark olfaction is fairly sensitive--although it varies by the species. They can detect very small amounts of a chemical within the water, which undoubtedly is advantageous for hunting. 

However, in order for the shark to actually detect said chemical (e.g., trace amounts of blood in the water), it needs to actually physically encounter it. 

So, if you cut your finger and start bleeding, it's not like sharks within 1+ kilometer distance will all start swimming towards you because they haven't actually physically encountered blood in the water. It would require a steady stream of blood leading from you to it. 

Another way to think about this is how we detect stars in the sky with our vision. If a star forms today, and it's 1 million light years away from earth, we won't know that the star has formed until light rays travel far enough to reach earth (i.e., 1 million years from now).