They all do it, apparently it's because their motor skills initially aren't developed enough to use their fingers to learn texture. Putting things in your mouth provides more data so to speak.
Mine also really liked eating sand for a while :-)
I believe it's much simplier: since eating is primary for any living species, how would you differentiate food from non-food while understanding the world (and still need to eat)?
The problem with that is that if it isn't food they usually don't try to eat it. They just put the object in their mouths and kind of feel it around. There's no attempt to consume it.
