Optical illusion (invisible flask)
To make and see it with different fluids we need 3 cups and 3 flasks to compare effects. As your fluids you can use water, vegetable oil, and liquid glycerin. How to make: Place your flasks with water, vegetable oil, and liquid glycerin into your cups filled halfly with the same fluids in it accordingly.
What to know:
See the difference between effects. Your flask can get easily seen, or hardly noticeable or even invisible due to density and refraction. Optical Illusions can use color, light and patterns to create images that can be deceptive or misleading to our brains.
Optical illusions simply trick our brains into seeing things which may or may not be real.