Anything

two diagrams on a blue background: the top diagram shows a pendulum with a clock and a labeled length L, while the bottom diagram illustrates a light beam bouncing between mirrors in a triangular path, with a clock and a distance D. Across the loop, the clocks in both diagrams advance, and the bottom diagram's clock shows a time difference that increases, eventually leading to the display of the equations Δt = 2L/c and Δt' = 2D/c.