Hal and Mab were so excited at hearing their father speak about a new secret, that they could hardly eat their supper. There were so many questions they wanted to ask. But they managed to clear their plates, and then, when Mr. Blake had on his slippers¹, and had put plenty of coal on the furnace, Hal climbed up on one knee, and Mab on the other.  
"Now, Daddy, please tell us the secret," begged the little girl.
 
"And tell us what makes water freeze, and how it gets cold, and what makes us warm when we run," added Hal. "Sammie Jones is going to ask his father what makes it lightning in a thunder storm."
 
"My goodness me sakes alive, and some peanut candy!" cried Daddy Blake with a laugh. "What a lot of questions!"
 
"But the secret first, please," begged Mab.
 
"Well, let me see if it is going to be cold enough for me to tell you," said Mr. Blake. "It must be freezing cold, or the secret will be of no use."
 
Daddy Blake went to the door, outside of which hung an instrument called a thermometer. I guess you have seen them often enough. A thermometer is a glass tube, fastened to a piece of wood or perhaps tin, and inside is a thin, shiny column. This column is mercury, or quicksilver. Some thermometers have, instead of mercury, alcohol, colored red, so it can easily be seen.
 
You see mercury, or alcohol, will not freeze, except in much colder weather than you ever have where you live, unless you live at the North Pole. Up there it gets so cold that sometimes alcohol will became as thick as molasses, and then it is not of any use in a thermometer. But mercury will not freeze, even at the North Pole.
 
The word thermometer means something by which heat can be measured.
"Thermos²" is a Greek word, meaning heat, and "Meter" means to measure.
Though of course a thermometer will measure cold as well as heat.
"Is it cold enough?" asked Hal, as Daddy Blake came back from looking at the thermometer.
 
"Not quite," his father answered. "But the mercury is going down the tube."
 
"What makes it go down?" asked Mab.
 
"Well, let me think a minute, and I'll see if I can make it simple enough so you can understand," said Daddy Blake.
 
Those of you who have read the other "Daddy" books know how many things Mr. Blake told his children, and what good times Hal and Mab had with him. He was always taking them somewhere, and often one or the other of the children would call out:
 
"Oh, Daddy is going to take us walking!"
 
Sometimes perhaps it might not be for a walk. It might be for a trip in the steam cars. But, wherever it was, Hal and Mab were always ready to go with their father.
 
In the first book I told you how Daddy Blake took Hal and Mab camping.
They went to live in the woods in a white tent and had lots of fun.
Once they were frightened in the night, but it was only because
Roly-Poly, their poodle dog—
But there, I'm not going to spoil it by telling you, when you might want to read the book for yourself.
 
In the second volume, called "Daddy Takes Us Fishing," I made up a story about how Hal and Mab went to the seashore cottage, and learned to catch different kinds of fish; even the queer, pinching crabs³, that turned red when you boiled them.
 
Once Mab fell overboard, and the children nearly drifted out to sea, but they got safely back. After that they went to the big animal show. And in the book "Daddy Takes Us to the Circus," I told you how Hal and Mab were accidentally taken away in one of the circus wagons⁴, and how they traveled all night. And the next day they rode on the elephant's back, and also on a camel's and they went in the big parade. Oh! it was just wonderful the adventures they had!
 
Hal and Mab lived with their papa and mamma, and Aunt Lolly, in a fine house in the city. But they often went to the country and to other places where they had good times. In the family was also Uncle Pennywait. That wasn't his real name, but the children called him that because he so often said:
 
"Wait a minute and I'll give you a penny."
 
Hal and Mab used to buy lollypops with the pennies their uncle gave them. And then—Oh, yes, I mustn't forget Roly-Poly, the funny, fat, poodle dog who was always hiding things in holes in the ground, thinking they were bones, I guess. Sometimes he would even hide Aunt Lolly's spectacles and she would have the hardest work finding them. Oh, such hard work!
 
"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 swell⁵ up, and get larger. That's why the steam from hot water swells⁶ up and makes the engine go, and pull the cars.
 
"And in hot weather the mercury swells, puffs⁷ 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 cork⁸ 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!"
 
You may be sure Hal and Mab did not want to go to bed, even if they were sleepy. They wanted to stay up and watch the water in the bottle freeze. But Mamma Blake soon had them tucked snugly⁹ under the covers.
 
Then Daddy Blake fixed¹⁰ the furnace fire for the night, as it was getting colder and colder. Next he opened a package he had brought home with him. Something inside jingled¹¹ and clanked, and shone in the lamplight as brightly as silver.
 
"What have you there?" asked Aunt Lolly.
 
"That's the children's secret," answered Daddy Blake, as he wrapped the package up again.
 
Hal was up first in the morning, but Mab soon followed him.
 
"Daddy, where is the bottle?" called Hal.
 
"May we get it?" asked Mab.
 
"Oh, it is much too cold for you to go out until you are warmly dressed!" cried Daddy. "I'll bring the bottle in so you can see it."
 
He went out on the porch in his bath robe and slippers, and quickly brought in the bottle of water he had set out the night before.
 
"Oh, look!" cried Hal.
 
For the bottle was broken into several pieces, and standing¹² up on the board on which it had been set, was a solid, clear piece of ice, just the shape of the glass bottle itself.
 
"Oh, somebody broke our bottle!" cried Mab. "Now we can't hear the secret!"