LOL why don't you take 10 pieces of whatever and we'll call each one a share cert and you own them all.
Now I borrow 1. Now you have 9 and I have 1 = 10.
I sell the 1 I have to Joe. Now Joe has 1 and you have 9 for a grand total of 10.
Now I owe you 1 share and since there are only 10 shares in total, I now have to buy 1 from either you or Joe to pay you back.
So I buy 1 from Joe and give it to you. Now Joe has 0, I have 0 and you have 10...how about that.
Or we'll do it the other way, I buy 1 from you, now you have 8, I have 1 and Joe has 1 ....dang it's still just 10 and I owe you 1...so I give you the 1 I have....now I've got 0, Joe has 1 and you have 9...dang it's still 10 total.