besides
preposition
in addition to someone or something; except for someone or something
We have lots of things in common besides music.
Besides working as a doctor, he also writes novels in his spare time.
I’ve got no family besides my parents.
I’ve got plenty of other things to do besides talking to you.
beside
preposition
1 next to or at the side of someone or something
He sat beside her all night.a mill beside a stream
2 compared with someone or something
My painting looks childish beside yours
.IDIOM
beside yourself (with something)
He was beside himself with rage when I told him what I had done.They were beside themselves with excitement.
Which Word: beside 
besidesThe preposition beside usually means “next to something/someone” or “at the side of something/someone”: Sit here beside me.Besides means “in addition to something”: What other sports do you play besides hockey? Do not use beside with this meaning.The adverb besides is not usually used on its own to mean “in addition.” It is mainly used to give another reason or argument for something: I don’t think I’ll come on Saturday. I have a lot of work to do. Besides, I don’t really like parties. She likes football. Besides, she likes tennis and basketball.
MORE INFO : http://oaadonline.oxfordlearnersdictionaries.com/dictionary/beside
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