Try this classic trick at home with different shaped glasses (which shapes the water within them) and a few patterned backgrounds. The Things made of cardboard YouTube channel demonstrates in the video above.
As you look through the glass, you’ll see the background pattern shift. Why? As the light passes through the water, it bends, distorting the images and objects behind it. From PhysicsClassroom.com:
As light travels through a given medium, it travels in a straight line. However, when light passes from one medium into a second medium, the light path bends. Refraction takes place. The refraction occurs only at the boundary. Once the light has crossed the boundary between the two media, it continues to travel in a straight line. Only now, the direction of that line is different than it was in the former medium.
Read more for additional info and experiments.
Pair this video with another classic water glass experiment: The Reversing Arrow Illusion.