As Peter Parry would probably point out, it's at this point you discover that the gate valve that shuts off the supply from the hot tank to the kitchen tap is seized solid.
You decide to be brave and whip out the old one and screw in the new using an old towel to stop the hot water splashing around but you cross thread the new one in the process.
By this time hot water at 60 Centigrade has made it to the tap and you cannot screw or unscrew the new ceramic unit, while hot water is pissing everywhere all around you .....