system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is a leech or tick able to get lots of blood from anywhere, but a nurse with a needle has to hit a vein?
gpt
The nurse with the needle is trying to get a lot of blood. Poke your finger with a small needle and a drop of blood will come out. A drop is quite a lot for a small animal.

(also, the saliva of these animals has anti-clotting agents preventing the wound from closing, keeping the blood flowing)