Einstein was born in 1879 in Germany. As a child, he was slow to learn to talk. As a pupil, he was backward. But when he was fourteen years old, he became clever. He taught himself maths from textbooks. He studied hard because he wanted to be a physicist. In 1901, Einstein began teaching. In 1902 he continued his studies at the University of Zurich. Several years later, he formulated his famous Theory of Relativity .To most people the law of relativity is difficult to explain. But once Einstein explained it very well to a group of young students. He said, "When you sit with a good girl for two hours, you think it is only a minute. But when you sit on a hot stove for a minute, you think it's two hours. That is relativity." After Hitler came into power in Germany, Einstein went to America. In 1940 Einstein became an American citizen. In 1955, Einstein's life ended at the age of seventy-six. The law of relativity is  _   .
very hard for most people to understand too hard for Einstein himself to explain too hard to explain to anybody except Einstein easy to understand only for the young students