system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do most animals have two testicles?

I mean just one would work fine. Why does nature go through the effort of making another one?
gpt
Redundancy.

Having two of a thing makes it less likely that you'll die if one breaks. Two lungs, two kidneys, two eyes, two limbs (or each kind), two ears, etc. Some organs are so central or complicated that having two isn't practical (stomach, brain, heart, uterus's), at least in humans.

Evolution is going to be \*really\* concerned with anything related to reproduction. So testicles are going to be really high on the evolutionary selection list to make sure they work. Redundancy is a good way to help with that. We also have a \*very\* extreme pain response to any damage in that general area to condition us to not get into situations where we might damage one, let alone both.