The neat thing about science is that it isn’t stupid and figuring out how to do this experiment is as important as finding the results of using it.

http://www.isd77.k12.mn.us/resources/cf/SciProjInter.html

water! take two soda bottles (preferably about 2liter sizes), fill one with water, join them together at the necks with a strong tape. when you invert one, the water will spill into the one below. flip it over and it does it again. time it to see how long it takes the water to go from the top bottle to the lower one and that is how your “hour glass” is timed. I.E. if it takes 20 secs for the water to go through, then you have a 20sec timer.

side note, this is usually used for “bottle tornados” but you get the same affect. You could slow or speed time by making using thicker liquids like corn syrup and another with water.

Don’t wait until the last minute to start a project like this.