Justin Bieber — Pelted By Water Bottle in Brazil

Justin Bieber took a water bottle to the HEAD during a live concert in Brazil this weekend … just like he did in 2010 … but this time, he reacted MUCH, MUCH differently.

You remember the 2010 shot, when someone beaned the cute, adorable (shirted) pop star — and he reacted with by smiling and telling the crowd, “Ow! That didn”t feel too good.”

But Brazil was different — an unshirted Bieber got smacked (while singing “Baby”) … and reacts by mad-doggin” the crowd and storming off the stage.

How quickly they grow up …