Wanna watch a pyrex dish and some water make a penny disappear? How about a little more water (in an unusual place) making it come back? Take a look at a confusing physics demonstration that you can do at home, whenever you want to frustrate your guests.

Light knows that the vast majority of the population is entirely dependent on it to get around. Sometimes it displays great noblesse oblige, giving us rainbows and sparkling diamonds. Most of the time, though, it takes advantage of our dependence by playing tricks on us. One of these tricks is shown in the below video.

In this video, a penny placed under a pyrex beaker disappears the moment water is poured in beaker. When looked at from the top of the beaker, the coin is visible. When looked at from the side, it's gone. If the penny is placed inside the beaker, and water is poured in, it's perfectly visible again. What's more, if the penny is gotten wet and put under the beaker full of water again, it's still visible, despite being invisible when the coin is dry. What's going on?

Massive differences in the index of refraction are what's going on. A material's index of refraction is a measure of how fast light moves through it. Light only moves at 'light speed' through a vacuum. Anywhere else it is effectively slowed down. If it were slowed down the same amount in all materials, we wouldn't see half the amount of optic phenomena we do. When light moves from one level of refraction to another, it bends. The relatively large difference in the index of refraction for water and air (1.33 versus 1.0), is why our fingers look distorted, or broken off from the hand, when we put them in water. Pyrex, the beaker that the penny is under, has an index of refraction of 1.47. That's much closer to water than it is to air.

So when the penny is at the bottom of the beaker, there's only one sharp turn that the light from the penny has to make, from the pyrex dish to the air. The water-to-pyrex transition is comparatively mild, with little bending. The penny is distorted, but it's visible. When the wet penny is beneath the dish, but under another layer of water, the light also only has one sharp turn â€” back into the air at the end of its journey. Before that it only travels through water and pyrex, which have similar indices of refraction, and so it isn't bent much.

When the penny 'disappears,' though, it is taking two sharp turns, the massive turns between the pyrex and the air both at the bottom of the beaker and at the side. And, because of the way light bends, both turns are in the same direction, away from the eye of the viewer. Imagine the beaker full of water like an immense piece of rectangular carpeting on the concrete floor, and the light like a person on roller skates. (No, seriously, this will help.) The viewer is on the right side of this carpet. In order for them to see the penny, just under the bottom of the carpet, the light has to get from the penny to them. That means it has to head up and right.