You may know that fiber optic cables gather up light and deliver it from one end to the other without letting it shine out the sides. But did you know that you can do much the same thing with water (and better with other liquids)?
In fiber optic cables, light curves with the cables, is shot into one end, survives its twists and turns, and comes out the other. For some time, fiber optics was held up as the technology of the future, but you can also recreate these cables using a stream of water. Grab a bottle of water and drill a hole in it. Turn the lights out, get a flashlight (or a laser pointer), and aim its light directly across the bottle at the hole.
You'll see some light shining down the stream of water. When you put your hand underneath the stream, the light should form a fairly perfect circle in your palm. This is due to internal reflection. When light travels between two media, it bends. Sometimes, when it goes from water to air, it just makes a turn and keeps going. Other time it gets reflected back into the stream and keeps going. The water isn't reflecting as much light as fiber optic cables do — if it were, we wouldn't be able to see the light in the stream - but enough of it is reflected to entirely "fill" the stream of water that hits your hand below.
Neat as this is, there is a way to make the "cable" more efficient. Instead of water, use propylene glycol. This is the main ingredient in food coloring, hand sanitizers, and saline solutions. It has a greater index of refraction, compared to air, than water does. It's hard to stop light if it hits a surface head on. The greater the angle that the light hits the surface at, the more likely it is to be reflected. Media with a big difference between their respective indices of refraction will reflect the light at smaller angles than those with small differences. With its large jump, propylene glycol streams won't glow like water ones. Only a tiny thread of light can be seen in the propylene glycol streams, and the light at the "other end" of the stream is much brighter.
In fact, propylene glycol is a better reflector than fiber optic cables themselves, but it's a liquid, and so not well-suited to the electronics industry. Still, if you have to communicate in Morse code down a waterfall, this is the way to do it.