Purchase Order to Bill

Purchase Order to Bill

When a Purchase Order that I created with one of my vendors has been received and I want to pay them, I go into the Purchase Order and click Convert to Bill.  But none of the information from the PO goes to the Bill.  Am I doing something wrong?  Shouldn't the item(s) on the PO be put on the Bill?  I shouldn't have to re-enter items on the Bill should I?
-Tom