In 2014, NPR posed a challenge to its audience: if they listened to clips of hot and cold water being poured into glasses, could they tell which was which? The test was inspired by a similar one performed by British sensory-branding company Condiment Junkie, and the results were the same in both cases: overwhelmingly, people could tell which was which. In the NPR test, 80% of people correctly guessed the cold clip and 90% correctly guessed the hot clip. (Try the test yourself by watching the first video below).
It all comes down to water's viscosity. Cold water is more viscous than hot water, which means that its molecules more readily cling to one another. This makes it relatively sticky, enabling it to bead on a windshield or stick in the bottom of a straw. When water is heated, the water molecules begin to move around much more rapidly, keeping them from clinging together and making the water runnier—that is, less viscous. Water with less viscosity will splash in smaller particles, and smaller particles create a higher pitch when they hit the glass. Explore the science of water temperature with the videos below.