"Well, Daddy," asked Mab, after Mr. Blake had sat silent for some time, "have you thought of a way to tell us what makes the shiny stuff in the—in the—in the—Oh! I can't say that big word!" she finished with a sigh.
"The mercury in the thermometer!" laughed Daddy Blake. "You want to know what makes it go down? Well, it's the cold. You see cold makes anything get smaller and shrink, and heat makes things swell up, and get larger. That's why the steam from hot water swells up and makes the engine go, and pull the cars.
"And in hot weather the mercury swells, puffs itself out and creeps up inside the little glass tube. In winter the mercury gets cold, and shrinks down, just as it is doing to-night."
"But will it get cold enough so you can tell us the secret?" Hal wanted to know, most anxiously.
"Perhaps," said his father. "We will try it and see. I will fill a bottle with water, and we will set it out on the back porch to freeze. If it freezes by morning I will know that I can tell you the secret."
"Oh, do we have to wait until morning?" cried Mab, in disappointed tones.
"That won't be long," laughed her father. "You can hardly keep your eyes open now. I guess the sand man has been here. Go to bed, and it will soon be morning. Then, if there is ice in the bottle, I'll tell you the secret."
Daddy Blake took a bottle, and filled it with water. He put the cork in tightly, and then twisted some wires over the top.
"What are the wires for?" asked Hal.
"So the ice, that I think will freeze inside the bottle, will not push out the cork," explained Daddy Blake. "Now off to bed with you!"