Essential Parts

BF, "Go ahead, turn on the faucet."
TMF, "Okay.  I'm so excited to use the faucet for the first time!"

TMF, "Uh oh, what was that noise?"
BF, "What noise?"
TMF, "It sounded like there was a clank."

TMF opens the sink cabinet door,  "Oh no!  There's water leaking!"

After inspecting the sink cabinet for a few seconds, BF simply says, "Oh, that's what that part is for."
TMF, "WHAT?" 

TMF, "How could you leave OUT a part?"
BF, "I didn't think it was essential, but apparently it is."
TMF, "What?
BF, "Oops."

Apparently the essential part kept the water hose connected.  That's kinda important.