When we run, or saw wood, our muscles contract, and need more food-fuel to burn, and pour more waste-stuff into the blood to be thrown off through the lungs; so the heart has to beat harder and faster to supply these calls.

When our stomach digests food, it needs a larger supply of blood in its walls, and the heart has to pump harder to deliver this.

Even when we think hard or worry over something, our brain cells need more blood, and the ever-willing heart again pumps it up to them.

This is the chief reason why we cannot do more than one of these things at a time to advantage.

If we try to think hard, run foot races, and digest our dinner all at one and the same time, neither head, stomach, nor muscles can get the proper amount of blood that it requires; we cannot do any one of the three properly, and are likely to develop a headache, or an attack of indigestion, or a "stitch in the side," and sometimes all three.