One of the most enigmatic particles of the subatomic world is the so-called neutrino. The neutrino comes in three flavors and plays a central role in the physics of one of the four fundamental forces of Nature - the Weak force. It is prevalent in many radioactive processes and exists copiously all around us. For example, our Sun spits large amounts of neutrinos at us. As you are reading this, neutrinos are passing through your flesh and bones like knife through butter… if you feel a tingling, it's either neutrinos or the broccoli you ate yesterday.
Two interesting things about neutrinos set them apart from the other particles of the subatomic world. First, they are extremely light. In fact, they were thought to be massless for a long time. Only recently was it discovered that they have tiny masses. Tiny numbers in physics are psychologically troubling. Usually, when something is extremely different from everything else around it - like very very light neutrinos compared to protons and electrons - you would worry that something deep is at work, something that you need to understand better. The extreme lightness of neutrinos is such a philosophical puzzle: it doesn't point to an actual problem in our description of the physics, but it suggests that our description may be incomplete or missing a deep principle at work.
The second unusual attribute of neutrinos has to do with the weakness of their interaction with the rest of the matter in the universe. Neutrinos travel near the speed of light and just go through virtually everything in the universe. To neutrinos, we and the Earth are mostly transparent. It is hence very difficult to detect neutrinos. Physicists have to build large underground tanks filled with water and live like hermits in caves hundreds of meters below the surface to detect neutrinos… at least that is the official excuse for this lifestyle.
Neutrinos also play an important role in understanding the cosmological history of the universe. They may for example be the source of the mysterious dark matter that pervades galaxies. And they are crucial for understanding how our Sun powers itself. The first video accompanying this post is a short one and talks about the basics of neutrinos - particularly the ones coming from the Sun. The second video is rather lengthy but very well done; and it goes into the physics of radioactivity and neutrinos in great detail - for those of you trying to understand that tingling you feel now and then...