Here’s a fun optics experiment, based on one created in 1842 by Jean-Daniel Colladon, that you can do at home using a laser pointer and a bottle of water.
Parental supervision required. (Eye protection recommended for kids.) Never shine a laser pointer at or near eyes, as it can cause blindness.
Skullsinthestars.com has a well-written, detailed explanation of how light can be reflected in a transparent medium, like water.