riddle i cant find answer to

I am found on every playground.
I am found in every office building
I am long... but also short
I am fat but sometimes skinny
I make dogs bark
And fat people cry
You can place a bet on me
I have more numbers than a phone book
I am lighter than a feather... but i have enough strength to kill a black bear..
Colleges despise me and preschoolers love me...