Daddy Blake and the two children glided1 to and fro over the ice of the frozen2 lake on their sharp steel skates. Soon all their cheeks were red and rosy3, and they felt as warm inside as though they had taken some hot chocolate at the corner drug store.
"Daddy," asked Hal, "what makes you warm when you run fast, or skate?"
"It is because your heart pumps so much more blood up inside your body," explained Daddy Blake. "Our blood is just the same to our bodies as coal is to a steam engine. The more coal the fireman puts under the boiler4 (that is if it all burn well, and there is a good draft) the hotter the fire is, and the more steam there is made."
"Is our blood like steam?" asked Mab, as she tried to peep down at her red nose and cheeks. But she could not see them very well so she looked at Hal's.
"Well, our blood is something like steam," said Daddy Blake, with a laugh. "That is if we didn't have any blood we could not move around, and live and breathe, any more than an engine could move if it had no steam.
"You see we eat food, which is fuel, or, just what coal and wood are to an engine. The food is changed into blood inside our bodies, and our heart pumps this blood through our arteries5, which are like steam pipes. Our heart is really a pump, you know; a very wonderful pump."
"My heart is pumping hard," said Hal, putting his hand over his thumping6 chest.
"Well," went on his father, "the reason for that is, that when we run, or skate fast, our body uses more blood, just as an engine which is going fast uses more steam than one going slowly. The heart has to pump faster to send more blood to our arms and legs, and all over, and whenever anything goes fast, it is warmer than when it goes slowly.
"If you rub your finger slowly over the window-pane, your finger will not be very warm, but if you rub it back and forth7 as fast as you can, your finger-tip will soon be almost warm enough to burn you.
"That is something like what happens when you run quickly. The blood goes through your body so much faster, and your heart beats so much harder, trying to keep up, that you are soon warm. And it is a good thing to exercise that way, for it makes the blood move faster, and thus by using up the old blood, you make room for new, and fresh.
"But I guess we've had enough talk about our hearts now," spoke8 Daddy Blake with a laugh. "We'll eat some lunch and then take home our fish."
Daddy Blake built a little fire on the shore, near the frozen lake, and over this blaze9, when the flames were leaping up, and cracking, he heated the chocola............