Preparation of the trick (must be done before the audience is present): Prepare a container with vegetable oil. Inside this container should be a Pyrex glass test tube.
Show small pieces of Pyrex glass and point out that they are from a test tube that has broken, something that often happens in a school laboratory. Point out that we can, with a magic liquid, get our test tube back in one piece.
Insert the small pieces of Pyrex glass into the container with vegetable oil. The audience will see the pieces 'disappear'. You can then ask the audience to help you with a magic word to get the tube back intact. Then remove the test tube that was inside the vegetable oil.
By adding vegetable oil to these Pyrex glasses, we will witness the light changing direction, also known as refraction.
When light travels from one medium (like air) to another medium with a different refractive index (like pyrex glass or vegetable oil), its speed changes, causing the light rays to bend, change direction. This bending of light is what we refer to as refraction. Refractive index is a measure of how much light slows down when passing through a particular material. The higher the refractive index of a material, the slower light travels through it, and the more it bends.
The index of Pyrex glass is similar to the refractive index of vegetable oil, so the light will have the same speed in both media, it will not bend when passing from the Pyrex glass to the vegetable oil and vice versa, which gives us the feeling that there is no other Pyrex glass inside of vegetable oil.