Have you ever wondered how brass instruments work? This Ted-Ed video shows you the physics of it all, from the air stored in the lungs, to the vibration of the lips, to the sound coming out of the bell.
Depending on the combination of the valves pressed (or position for instruments using a slide), they can alter the length of their tube, changing which harmonic series it belongs to. From there, what dictates which note in the harmonic series (also known as a partial, or overtone) is being played is all dependent of the lips of the musician and how fast they vibrate.
For a more in-depth explanation, watch the video!