Why does a child bites?

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My 2 half yrs old boy is always fighting with his 5 yrs old brother which is common i guess lol.Kids will be kids you cant stop them but well, last week he bites his older brother's back and left a nice teeth mark there.He had never done that nor has his older brother ever did that to him. Where did he learn it from? I told him not to do it and since then he has done it again this time on his brother's arm.What can i do to stop this? Dont tell me to bite his hand someone advise me to do that hahahaha