"Jane," asked Uncle Robert, "have you a candle?"

"Here is one, sir," said Jane, taking a candlestick from beside the
clock on the shelf.

Uncle Robert lighted it and held it near the window, just below the
sill. The flame flickered as the air from the window struck it, and then
turned straight into the room. He raised it just above the opening.
Instantly the flame pointed toward the window, but it did not flicker as
it had when held below the sill.

"The air must be going out up there," said Frank, "but it doesn't blow
so strongly as the air coming in."

"The air that comes in is cooler than the air that goes out," said
Donald.

"What makes the water boil?" asked Uncle Robert, turning to the kettle
on the stove, which had now begun to sing.

"Why, the heat, of course," said Donald.

"What raises the lid?" asked Uncle Robert.

"The kettle is too full," said Frank. "It is going to boil over."

"Why didn't the water run over when it was cold?" asked Uncle Robert.
"The kettle didn't seem full then."