The vibrating air column. Least water to most water will be high to low pitch. So the air will make the plastic vibrate when it goes by. Students may have various hypothesis on this but it must relate to the length of the vibrating air column and the pitch. The shorter the straw the higher the pitch.
Various instruments use holes in the vibrating air column to shorten and lengthen the air column.
Trombone is a good example. The tuning fork would be moderately loud, may be difficult to hear in the room. Times will vary but it should be less than the time in question 4. The tuning fork should sound louder now as it moves through a solid. The larger tuning fork should make a bigger splash.