system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Stem cells. The science and the controversy.
gpt
Stem cells can grow to be any cell you want. If you need new cells for your heart, brain, liver, whatever, these cells can be used as a base for growing new parts or repairing old ones. 

The controversy is with embryonic stem cells, which are only found in a human embryo that is 4-5 days old. To gather these cells, the embryo must be killed. The controversy is that some people are essentially saying that this is murder. Some people saying that it is not. 

Scientists are working on a better way to get these stem cells, partly because of this controversy, and because an embryo is really small and you can't get much out of it for the effort it takes (concieve a baby and all).