"Wouldn't that be a big thermometer!" exclaimed Susie. "Now put the thermometer on the floor," said Uncle Robert. "It's seventy-two degrees now," said Donald in a few minutes. "Let's put it on the broom," said Susie, "and hold it up to the ceiling." "It's warmer up there," said Frank, looking at the little gray cylinder when they brought it down. "It is six degrees higher than it was on the floor." "Why?" asked Uncle Robert. "The heat must go up there," said Donald. "It goes into the next room when the door is open," said Frank. "Does it go outdoors?" asked Uncle Robert. "Let's open the window and see," said Susie. Frank opened the window, but, instead of feeling the warm air going out, he felt the cool air coming in. "Uncle," asked Donald, "isn't the room full of air already?" "Yes," answered Uncle Robert. "Then I don't see how any more can come in at the window." "Are you sure none goes out?" "I could feel it coming in," said Frank. "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." "Somehow it seems to get more than full when it boils," said Donald. "See, it is boiling over." Just then Jane took a pan of apples out of the oven. Each one looked like a small volcano. "What happens to the apples when they bake?" asked Uncle Robert. "They just swell up so big their jackets won't hold them," said Donald, laughing. "It is heat that makes the bread rise, isn't it?" asked Frank. "Of course," said Susie. "Don't you know sometimes if the bread doesn't rise, mother says it is because it is too cold?" "There is something besides heat that makes the bread rise," said Uncle Robert. "Yes," replied Susie, "the yeast; but it must be warm--I know it must." "It seems as though everything is bigger when it is hot than when it is cold," said Frank. "And now I believe I understand something that happened not long ago." "What was it?" asked Uncle Robert.