Nothing. No layers present. The hot and cold water simply mixed together. We heated the hot water to a boil, getting it as hot as possible. We could have made the cold water colder by dropping ice cubes in it instead of using water straight from the tap. It's also possible I was too rough in pouring the water, causing extensive mixing. Regardless, we were not successful.
We were determined to see something, however, so we mixed water and oil and discussed why there is a clear difference in the water versus vegetable oil layers.
The oil is less dense than water and rises to the top, creating a distinct layer. We put some food coloring in the oil, which doesn't mix as it does in water. The food coloring drops stayed as drops in the oil which was kind of cool to see!