These articles survey two ideas Einstein discovered in 1907 and found very useful for his final discovery of the theory of general relativity in 1915. Both articles assume very basic high school physics knowledge as covered in “The free fall and Newton’s second law.” The second article assumes the knowledge of the first article, familiarity with the constancy of the speed of light, in addition to very basic high school physics knowledge. For an alternative approach to the second article, please read “Why time goes more slowly at a lower place and what a black hole is” listed in the section “Black hole.”