How to drink tap water from Rome’s public fountains

Before going to Rome I wondered, “Is it safe to drink the tap water in Rome?” And as I found out, not only is it safe, the tap water in Rome is quite tasty.  And, Rome is full of these public “Nasoni” drinking fountains.  At first I was just filling up my water bottle at the fountains, but then I found out about the hidden “drinking fountain” feature!