"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 arteries, 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 thumping 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 forth 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," spoke 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 blaze, when the flames were leaping up, and cracking, he heated the chocolate he had brought. Then it was poured out into cups, and nice chicken sandwiches were passed on little wooden plates.
"Isn't this fun!" cried Mab as she sipped the last of her chocolate.
"Indeed it is," agreed Hal. "I'm coming skating over to this lake every day!"