How did Moaning Myrtle Make the Water Splash?

[Robert Jones]

No: HPFGUIDX 93348

Here is another trivial problem I don't understand: In COS, Moaning 
Myrtle made a splash in her restroom when HRH were there — but how 
did she move the water?  I mean the ghosts at Hogwarts can walk 
through the walls and floors and cannot move anything.  When Nearly 
Headless Nick needed to make a commotion to help Harry, he had to 
get Peeves the Poltergeist to push over a cabinet — Nick couldn't 
move anything.  Also in the Deathday party, the ghosts couldn't even 
smell or taste the food when they passed through the tables, let 
alone move them.

So how Moaning Myrtle, a ghost, make the water move?  (Or for that 
matter, how does she remain cozy in her favorite U-bend in the 
pipes?  Shouldn't she just pass through the sides of the pipe when 
she wasn't looking, as it were?)

Bobby