y," 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 boiler (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 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!