system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why can't we produce blood and have to donate it from real people?
gpt
Just because it has one name doesn’t mean it’s one thing. Like a car is make up of lots of parts, Blood is also made up lots and lots of different pieces that are super tiny, so small, you can’t even see them, and they have to be fit together in exactly the right way for it to work. It’s still too hard for humans to put the pieces together in exactly the right way, but we’re working on it.