"Does the air in the bottle pull the rubber in with it?" asked Susie.
"But, Uncle Robert," said Donald, "what if wagon tires, apples, and air do swell up when they are hot? I don't see what all that has to do with the thermometer."
"I think I see," said Frank. "Why wouldn't this gray stuff in the thermometer get bigger when it's hot, if everything else does?"
"What is it that moves up and down in the thermometer?" asked Susie.
"It is mercury," answered Uncle Robert, "which is sometimes called quicksilver."
"It looks like silver," said Susie, examining it closely.
"Perhaps you can see this better," said Uncle Robert, taking a small bottle of mercury from his pocket and pouring a little into Donald's hand.
"How heavy it is!" exclaimed he, letting it roll about. "It feels just like lead."
"It is almost twice as heavy as lead," replied Uncle Robert.
"Put it in my hand, Donald," said Susie. "There, you've spilled it on the floor! Just see it run around!"