Black Bears and Grizzly Bears

How do you tell the difference between a Black Bear and a Grizzly?

If you see a bear, run to the nearest tree and climb it as fast as you can.  If the bear climbs the tree and eats you, it is a Black Bear.  If the bear hugs the tree, rips it out of its roots, slams you over the head with it, and eats you, it is a Grizzly.