What's the difference between "because" and "so"?

Because + reason
A: I’m going to buy gloves.
B: Really? Why?
A: Because it’s cold outside.

She runs once a week because she wants to stay healthy.
He didn’t go out because he didn’t feel good. 

So + result 
A: It’s cold outside today.
B: So?
A: So, I’m going to buy gloves.

She wants to stay healthy, so she runs once a week.
He didn’t feel good, so he didn’t go out.

Commas: Use a comma before the “so” clause.
X – She wants to stay healthy so she runs once a week. 
O – She wants to stay healthy, so she runs once a week.

Because can also start a sentence. Use a comma.
Because it was so hot, I turned on the air conditioner.
Because of the train delay, she was late for work.

BE CAREFUL
“Because” is used to join reasons and results. The following examples are not correct. They only show the reason and we don’t know the result.  
X – Because he wants to stay healthy. 
X – Because she wants to save money. 
X – Because they want to increase sales. 

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