If you want to save time, basically just mix tonic water and corn starch together. Potato is loaded with starch. All he did was show you how to extract it.
Corn or potato starch and water is a non-Newtonian fluid, meaning that the substance is unstable and easily and quickly shifts between being a solid and a liquid. This can be seen in several other combinations of substances, but starch (potato or otherwise) is the easiest combination to see this in action. The tonic water addition is pretty neat.