Good job!...
"RIVER" is the correct answer.

"What does that mean?" asked Trevor.

"I think I know" said Melody. "It's a river. The bed of a river is the very bottom, so in the desert, where there's no water, sometimes you can see a dry river bed where a river used to be. A river's bank is the land beside the river, like the shore of a lake. Finally, the mouth of a river is the place where it goes into the ocean."

"Hey, you're pretty smart!" Trevor said, and Melody smiled."But I have another question. Why do you think she wanted to tell us the word river?"
"I don't know why" Melody said, and then the lights came on again.

When the lights came on they saw their teacher walking toward them. "I'm glad you two din't get lost" she said. "This museum was built more than a hundred years ago. It was used as a hospital once during a war. Some people say they've seen the ghost of a woman who died here in those days. I don't think ghosts are real, but there are many old rooms and halls that no one ever goes into."
"Is the a river near here?" asked Trevor.
"No, but there's an old painting of one in that room over there."

Trevor and Melody went into the room that their teacher had told them about.