I read this on the wikipedia page for tap (valve):
At intermediate flow settings the pressure at the valve restriction drops nearly to zero from the Venturi effect; in water taps, this causes the water to boil momentarily at room temperature as it passes through the restriction. Bubbles of cool water vapor form and collapse at the restriction, causing the familiar hissing sound.
I don't understand why the water boils when going through the tap. Could someone explain it in more detail?