A sin is something that a religion tells its believers is a bad thing to do.
Religions that believe in God usually say that God told them that some things are sins. Usually the sacred books of a religion talk about sins. There are some things that most religions agree are sins. For example, almost every religion agrees that it is wrong to kill people (murder).
There are other things that are sins in only a few religions. For example, Muslims and Jews say that it is wrong to eat pork, but other religions do not think that it's wrong to eat this.
Some people are not religious. They say that people should try to do the things that help other people, and not to do the things that hurt other people. People who are not religious do not usually say that the things that hurt other people are "sins", but sometimes they use this word for these things.
Most religions describe "sins" as actions that break their rules.

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