There are two types of waves they are not teaching about in school. Gravity and electrical. Both classical (Newtonian) mechanical and electricity. Gravity and purely (non electro-magnetic) electrical waves are explained by first, Newton's universal attraction law in case of gravity and attraction and repelling force in case of electrical. Very simply, if you move any mass or electrical charge it will exert a variable force towards anything else that has a mass or electrical charge in the Universe, but that force is maximum near the source.
Let's start with a simple gravity pendulum
https://en.wikipedia.org/wiki/Pendulum_(mathematics)#Simple_gravity_pendulum
For the purpose of this demonstration we will neglect the gravity force which actually moves the pendulum that manifests on the vertical. It could have been a spring or a motor or whatever and should consider only the gravity force developed by the bob m1 interacting with any other stationary bob m2 at a distance (X zero) from the center of the pendulum on the horizontal.
This could be also be proven, practically. If we placed two pendula of the same length, weight, etc. in boxes so we can eliminate any acoustic phenomena or even in vacuum and move one of them, we would see, after a while, the other starting moving, so we could be sure the only way for this to happen is through gravity waves.
