Video: Watch this demo
Description: This is a Variable resonance tube. By changing the water level, you change the air column length.
With a turning fork, you can find the resonant frequency by changing the water level and listening for the sound to get very loud (within 2mm).
The smallest air column is the fundamental frequency, 1/4 of a wavelength.
If you can find the second resonant frequency, then the difference between them is half a wavelength.
If you know the frequency, and you found the wavelength, you can estimate the speed of sound in air.
Note: A pure tone driven speaker can also be used instead of a tuning fork.
Food coloring should be added to the water to make it more visible to students.