As human beings are land animals their bodies are fitted to live on a stationary base. The sea is always in motion. Anything floating on a big body of water is constantly lifted and let fall and tilted at many angles. This disturbs the nerves of the eyes and the balance or equilibrium of the body, producing dizziness in the head and "sickness" in the stomach. It is probable, too, that the contents of the stomach are more or less shaken by the motion. Many people become just as "seasick" on rocking railway trains as on boats. And some cannot even bear the motions of swings and hammocks. But people differ very widely. Some are never made sick at sea, and others never get over the tendency, no matter how much they travel. Children rarely suffer from this malady. That is probably because they tumble about more than do grown people. It doesn't bother an active boy much to have his stomach rocked, or even turned upside down.