particles that bounce of also each other

Hello I am a newbie to blender,

I followed some particle documentation
Now I try to figure out how to set particles so that they each would behave like real little balls,
So they would not merge or be in the same position of eachother.
I like them to be well like footballs.

I manage to bounce them on other objects but not from eachother.

Any idea how to do this ?