What You Did
You made a musical instrument in the percussion family. Percussion instruments involve two elements being struck together to produce a sound. This includes drums, piano, maracas, or even clapping your hands together.
You should have noticed that different water levels make different notes.
If you filled all the glasses with a similar amount of water, the sound produced by each glass will make similar or identical notes
Test what effect certain proportions have on the sounds produced, for example, filling one glass all the way, one half-way, and one quarter-way (4:2:1)