When you pour water into a thermos bottle, the sound you hear changes with the condition of the water: the pitch is low, the pitch is high, and the tone is highest when you fill it up. The reason is simple: when the water is poured, the air in the bottle is vibrated and makes a sound. This part of the air is the sound source. At the beginning, there was a lot of air inside, and the air column was long. It vibrates slowly, the frequency is low, and the tone is low. The water is more and more filled, the air is getting less and less, the air column is getting shorter and shorter, and the air is short. The column vibrates fast, the frequency is high, and the pitch is also high.