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WHAT makes your watch go, grandpa? Please open it and let me look inside. I have a toy watch; but the bands stop moving as soon as I take out the key."

"You could not understand much about all these curious wheels, my dear; but I will tell you that there is one spring, called the mainspring, which keeps all the rest in motion. Did you ever think how much wonderful machinery must be in your body, to enable you to move your bands and feet?"

"Is my heart my mainspring, grandpa? I can feel it beat, like the watch ticking."

"Yes; you may certainly call it so; and when you are older you will learn its uses."