Schmutz....and why kids are drawn to it.

I have a question, and no, I don't expect any kind of reasonable answers...but why on earth do small children feel the need to pick up garbage and stick it right into their mouths?  I watch 8 month old twins.  I have been watching these babies since they were 2 months old, so I have had the pleaseure of watching them grow.  Well today I had the pleasure of following around the little boy and pulling all kinds of schmutzy stuff out of his mouth and it made me remember somthing that my youngest did...she was 3 and a half, maybe 4 and we were visiting Grandma on Cape Cod.  My little girl had something in her mouth and whe we asked her what it was she proudly pulled a peice of gum out of her mouth.  Now, I must tell you readers that I didn't give her any gum, nor did my husband or his mother so I asked her where she got it and my little angel took me by the hand and pointed to a beautiful shady spot on the sidewalk where she picked the pink glob up and popped it onto her mouth.  Where do they learn to do this?  I mean, I know that any level headed parent or even a really crappy parent would not intentionally teach their kids to eat rubbish, right?  So where do they learn this.  I know, I know, natural curiosity: touch, feel, taste, sight, smell...the five senses all need stimulation, but c'mom...doncha sometimes want to scream?  How many times a day can you vacuum or dry mop?  It is somethng that all small children do and they stop doing it around the time they go to preshool and it is frustrating.  But rest assured they all grow out of it and they will survive....what ever doesn't kill them makes them stronger!!!