When you tap a glass with a little water in it It has a high pitch when you tap a glass with water filled to the top it has a low pitch why does this happen?

Glass has a natural frequency at which it vibrates, known as its resonant frequency. If you put energy into the substance at its resonant frequency, you will force it to vibrate or resonate (resonance is a forced vibration). So, tapping imparts energy to the glass molecules and causes them to resonate. This motion sets up a wave of vibration traveling through the glass. The vibrating glass causes air molecules to vibrate similarly. The vibrating air molecules are the sound wave that you hear (the frequency or pitch of the sound wave is the same as the resonant frequency of the glass). As the resonant wave moves through the glass, it moves the water molecules with it, creating a wave of water that you can see near the edge of the glass. The dragging water molecules effectively increase the mass (both the water and the glass molecules) and reduce the energy of the wave traveling through the glass. When the energy is reduced, so is the frequency of the wave in the glass, which is reflected in the pitch of the sound wave that you hear.

In simpler terms, when you tap a glass with a lot of water in it, there are fewer vibrations because they have more trouble traveling through the higher mass. Thus, the lower pitch.