A trumpet makes sound when the musician makes a buzzing sound while blowing air through closed lips and into the mouthpiece. The air causes a standing wave vibration in the air column inside of the trumpet, which travels down the instrument and is then manipulated by the pressing of the keys.
A trumpet has three rotary valves that change the length of the tube when they are engaged. Pressing the valves lowers the pitch of the sound that comes out of the instrument. The very first trumpets were made from the hollowed-out tusks of animals. Modern trumpets are made of brass.