 And thinking about life, one of the things I really like to ask is, our virus is alive. So if we think about what it means to be alive, something that's alive has a structure. It has homeostasis, which is basically the ability to keep the inside of its structure at a somewhat constant, in constant conditions. It has a metabolism which allows it to extract energy from its environment to be able to grow and die if there's not enough energy. It loses its structure or can't maintain its homeostasis. And then it has a reproduction. It has to be able to reproduce with inheritance. So we can ask about the set of things with viruses. So viruses have a structure. I'll draw one. They have different shapes. I'll draw one that looks a little bit like a coronavirus since that's so important to us right now. It's roughly spherical with spikes coming off. It has an interior to the structure and inside of it is messenger RNA. And we'll talk about messenger RNA later, but it's basically a form of inheritance. So it has a structure and it has inheritance. And the interior of the virus is at fairly consistent conditions. So we can say that it has some form of homeostasis. However, the virus on its own does not have the ability to metabolize. On its own, it doesn't grow, but it definitely can die. For the coronavirus, the cell wall is made up of lipids, which are fats. So adding a little bit of soap just breaks open their walls and allows them to die. And they can't actually reproduce on their own either. These three properties, metabolism, growth, and reproduction, require another type of cell. These other cells can be bacteria or archaea, which are single-celled organisms. Or they can be eukaryotes, which includes fungi and plants and animals and a lot of the macroscopic life. So if we have the cell, the virus interacts with the surface of the cell. And for example, in the case of a coronavirus, these proteins interact with a receptor on the cell and the virus ends up going inside. So the virus ends up inside the cell, and the cell has its own ability to grow. So it has metabolism and reproduction. And the virus basically manipulates the cell's reproduction abilities to make copies of itself, and it gets the energy as provided from the cell. So the virus uses other cells to actually perform three of the key functions in terms of life, which is the metabolism and the reproduction. The virus in general stays the same size. The size is sort of set by how big it needs to be to package its messenger RNA. So if we get back to the question about whether a virus is alive or not, it really gets to the question of interactions. The virus can only perform the full functions of life if it has a host cell to infect. One of the interesting things, so a lot of times we don't consider viruses as being alive because they don't have their own ability to metabolize and reproduce. However, we can think about other life on Earth, and we as people cannot live without other organisms as well. We require other organisms to create a source of our food, to produce the oxygen that we react with, to provide all the materials and environments that allow us to grow and reproduce and get our energy. One of the questions is whether or not there is any life that can live alone. And the interactions between organisms may also be a universal characteristic of life. Thanks for watching.