Whoa.

I am taking a physical chemistry class, in which we are learning about quantum chemistry and quantum mechanics. Here is a story which I thought was incredibly cool.
Einstein proved that light (sometimes) acts like a particle. Some other people thought this was pretty cool, as light had always been thought of as a wave up until that point. So, they decided to do an experiment to support his theory. They got two pieces of material and cut two slits in one piece. The other piece was used as a recording board. Next, they took some equipment that would shoot photons (light) at the first piece of material. The idea was that the photons would go through the slits and make a pattern in the shape of the two slits on the back board, which is what particles should do (imagine shooting marbles at a board with two slits cut in it). Instead, they got an interference pattern, which is what waves do.
This was strange, so they decided see what happened if they turned the intensity way down, to one photon at a time. So, they would shoot one photon at a time at the board. They still got an interference pattern, which means the photon was interfering with itself--it simultaneously entered both slits.
This was very strange, so they decided to see why. They set up equipment to observe the photon as it came through the slits so they could see how, why and when it became a wave. The photons did not cooperate, and they got a pattern in the shape of the two slits. Watching for the wave caused the photon to behave only as a particle.
This was extremely strange. So they came up with quantum mechanics and wave-particle duality. Wow.