Does light change the amount of heat in a substance?
No; boiling water is as hot in the dark as it is in the light; and ice is as cold in the daytime as it is at night.
Is there any other source of heat?
Yes; heat is produced by rubbing or striking substances together.
What will result from rubbing two pieces of wood together?
They may be rubbed until they take fire.
How do Indians kindle fires?
By rubbing two pieces of wood together until they take fire.
How is heat produced by the brakes on railroad cars?
It is produced by the car-wheels rubbing against the brakes.