Feed me and I live. Give me drink and I die. What am I?

I hold things together, yet I’m gone in a snap, I fasten the pages, I shorten the gap. I capture a moment, then cut it away, Too small for a film, too long for a frame.

Teachers love me Doctors fear me Snakes of myth might slither near me Once a princess’s surprise I’m beheld by many eyes

When you need to rest or snooze, my four legs are yours to use. Potatoes spend too much time on me when they should be out and running free. Two notes make up my name, it's true, though I will never sing to you.

My first four a lie, a bogus farce, My last three something that comes from an arse. My whole dances foamily upon your top, Coaxing clean, shimmery waves from your filthy mop.