Even when you are only seeing one test tube disappear into this beaker, there are another two more inside. In fact, there's also a beaker hidden inside, completely invisible to the eye. How is this sorcery possible? SCIENCE.
It's a clever trick to try at home for those of you who never took chemistry classes: Get a glass recipient, fill it with glycerol or vegetable oil instead of water and put a piece of solid glass or another recipient full of glycerol inside. It will disappear instantly, right before your eyes.
The explanation is simple: Glycerol has a refractive index very similar to glass. Light doesn't change its speed going from glass to glycerol, there's no refraction and the glass becomes "invisible".