The more work done the more fuel needed.

That is what we mean when we talk about the food giving "working strength." A farmer and his wife and usually all the family need much fuel because they do much physical work.

Even people whose work is physically light require considerable fuel.

A quart of milk will give as much working force as half a pound of bread, one-fourth of a pound of butter, or six ounces of sugar.
And this is in addition to the other advantages already mentioned.
_Milk contains specifics for growth._ Experiments with animals have taught us that there are two specific substances, known as vitamines, which must be present in the diet if a young animal is to grow.

If either one is absent, growth is impossible.