In free space, they glide without friction, indifferent to the passage of time. In glass or water, they slow, hesitating, their wavelength shortening as if the medium were a crowded ballroom. Some materials sip energy, turning waves into heat; others bend and split them, revealing colors and hidden structure. Boundaries are drama: reflection sends waves recoiling like startled birds; refraction makes them change course, bending paths and altering tempo; at interfaces, waves can whisper secrets to each other, interfere with a delicate pattern of constructive crescendos and destructive silences.