 One of the requirements of life that we talked about was the presence of carbon and how important carbon is to life and many scientists think that carbon is required for life elsewhere in the universe. One of the common questions that we get asked is whether or not silica-based life is possible. So for carbon, I talked about how it makes complicated molecules like various ring structures or chains and each one of these points has carbon on it and those carbons can often bind to a hydrogen or an oxygen So I just made up this molecule. So I don't take it as something that would really form. So these molecules are reactive in a lot of different ways and if for example, I remove one of these hydrogens here the carbon can readjust maybe by forming a double bond here. The carbon likes having four bonds and also losing this hydrogen here. And that would change the geometry of the structure. Those complicated reactions with molecules are really important for creating enzymes for life. So let's take a look at silica-based life. So for silica it has a different structure. It tends to bond to four oxygens in a tetrahedron. So the silica is really connected to four oxygens. And so the reactive parts are really the oxygens that are on the outside of the silica-tetrahedra. So oxygen really likes electrons. Chemically it binds them very strongly and it needs two electrons in two bonds with something. And so if it's bonded to the silica in one place it might connect to another silica and the silica is always bonded to four oxygen. So one of the nice things about silica is that you can make complicated chains. So much like carbon you can get chains and complicated structures. The difference is that unlike with carbon, I can't just remove one of these hydrogens. If I try to remove a hydrogen the bond with the oxygen is really really strong and it needs to be replaced with something else. So something that maybe it can be replaced with might be a sodium ion or it could be replaced with a silica-tetrahedra. When I removed a hydrogen from the carbon atom I said well it'll maybe form a double bond with the carbon that doesn't happen with the silica-tetrahedra. The hydrogen needs to be replaced with something else. And so it's not as reactive and certainly not in the same way that carbon molecules are. And so you can get the complicated structures but life requires the ability to extract energy from its environment. It requires reactions to grow. It requires reactions to make a its molecules for inheritance. This lack of reactivity of the silica-tetrahedra relative to the way carbon behaves that makes us think that silica-based life is not likely. We don't know everything. So it's possible that silica-based life might exist somewhere. But basically the key thing is you can think about glass. Like window glass is made mostly of silica and glass doesn't react. And that's sort of the key intuitive reason that silica-based life is not likely in the universe. It just doesn't react in the same way even though it can create complicated structures. Thanks for watching.