Why the Rabbis made the edict above: Pesachim 120b-121a
Rabbinically, Hands are automatically a Second Descendant Level of Impurity: Pesachim 17b
Hands Receive impurity via liquids, Rabbinically: Pesachim 19b
Liquids do not make hands impure in the Temple: Pesachim 19a-b
Hands which worked with meat in which a Needle Which Contracted Impurity from a Corpse was later found, are not impure: Pesachim 19a-20a