How did Myrtle flood the bathroom?

Question

The Harry Potter and the Chamber of Secrets, Moaning Myrtle floods a bathroom because she's upset about a book being thrown at her.

Moaning Myrtle is a ghost, and as such shouldn't be able to interact with solid objects. So how did she flood the bathroom?

Answer 1

Myrtle can apparently interact with water.

There are several cases where she interacts with water:

Myrtle gave a tragic sob, rose up in the air, turned over, and dived headfirst into the toilet, splashing water all over them and vanishing from sight, although from the direction of her muffled sobs, she had come to rest in the U-bend.
_{Harry Potter and the Chamber of Secrets, chapter 9}

...

"Don't ask me," Myrtle shouted, emerging with a wave of yet more water, which splashed onto the already sopping floor.
_{Harry Potter and the Chamber of Secrets, chapter 13}

So apparently she can interact with water, although no explanation is given why.

---

Apparently, in the movie adaption, she does touch the taps, whereas in the books it doesn't show anything about the taps, as it directly states that the taps have never worked:

"That tap's never worked," said Myrtle brightly as he tried to turn it.
_{Harry Potter and the Chamber of Secrets, chapter 16}

So the movies shouldn't be taken seriously everywhere (they mess things up).

Answer 2

Ghosts can affect water, fire, and air.

In the JKR Pottermore writing on ghosts, it’s explained that though ghosts can pass through solid objects without having any affect on them, but they create disturbances in fire, water, and air.

Ghosts can pass through solid objects without causing damage to themselves or the material, but create disturbances in water, fire and air. The temperature drops in the immediate vicinity of a ghost, an effect intensified if many congregate in the same place. Their appearance can also turn flames blue. Should part or all of a ghost pass through a living creature, the latter will experience a freezing sensation as though they have been plunged into ice-cold water.
_{^{- Ghosts (Pottermore)}}

Myrtle could flood the bathroom because as a ghost, she was able to affect water, so could make the water from her toilet overflow.