Yell, thump, make noise, rise your hands and pretend to be bigger than you really are. Bears don't pick fights if they don't have to.
Unless the bear is eating a carcass. In such case, just leave silently.
Run downhill - the bear will have a disadvantage of worse balance.
That's from official advice of our National Parks.
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Let's not confuse being normal with being mentally healthy.
<not moderating PPR stuff concerning East Europe>