"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.
"Thermos" 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.