In the experimental setup, a cord with a smartphone as pendulum mass is attached to a spring pendulum (see Figs. 1 and 4). The vertical oscillation of the spring pendulum and the horizontal oscillation of the gravity pendulum alternate peri- odically. This is due to the fact that the resonance frequency of the two partial pendulums is calculated to be identical. One pendulum can excite the other and vice versa. The energy fluctuates between kinetic Fig. 1. Setup of the Metapendulum. energy and potential energy in the spring pendulum, and kinetic and poten- tial energy in the gravity pendulum. The motion can be char- acterized into three states (see Fig. 2, and video5)