Yes, you can really play music with just glasses and water. Here’s more from Scientific American:

The water on your finger, however, will allow your finger to slip, as it forms a cushion that reduces friction. When the pressure and amount of moisture are just right, this so-called stick-slip motion (the slight friction between your finger and the rim of the glass) will cause vibrations in the sides of the glass. The sides of the glass transmit the vibration to the surrounding air, creating a sound wave with a specific frequency. The frequency specifies the rate at which a vibration occurs and is usually measured per second or Hertz (Hz). There is a particular frequency, called the resonant frequency, at which the sides of the glass will vibrate most easily. The resonant frequency of wine glasses is typically within the range of human hearing (20-20,000 Hz), and this is why you hear the resulting resonant vibration as a tone. Now let’s make some music and you can play your very own wine glass instrument.

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