 Hello, my dog-pounds. Today we're going to talk about one of my favorite subjects, energy. Energy, if you remember, is one of the defining characteristics of living systems. Living systems can use energy. And so it's so important to me and so fundamental to understanding living systems and, like, why? Why are we doing any of this at all? We should actually spend an entire lecture just contemplating the idea of energy. And then we're going to spend two more lectures looking at how living systems acquire energy and then make use of it. We should probably start out with a definition of the word energy. And to help us with this definition, this is a picture of my young brother. He's 15 months younger than me. He was at his wedding and he was lip-syncing some song to his bride-to-be who you can see right here in the chair below him. I mean, he's not a singer. This was a very unique wedding, let's just say that. And he told me afterwards that he started to feel like, you know, the song was getting a little dry and a little boring so he decided to leap over the top of her head. And his biggest concern in this moment right here was making sure that his pants didn't rip. This is fantastic. Hopefully his wife was fully aware of what kind of man she was marrying. My brother has an unbelievable amount of energy. And I don't think anybody would argue that. But what we might actually argue about is, dude, what is energy? Of course, we need some kind of a definition. And I'm going to just take a second here to give us a definition if I can find my pen seriously. Where is my pen? There it is. I found it. It's all good. I hope this all works. Okay, energy. Whoa, that's not going to work. Really? Why won't it let me pen? I'm not going to pen. I'm going to get it ready and then I'm going to pen. But I'm going to tell you right now the energy. Okay, I'm going back to my brother then. I'm going to write it down. Energy is one of the fundamental things in the universe and I find it really hard to define. It is, from a physics perspective, energy is the ability to do work. But what is an ability? It's why we're going to spend an entire lecture talking about it because how do you even, how do you grasp the concept? If it's the ability to do work, there is essentially doing work right now on his own body and we can accept that yeah, it requires energy to make that happen. So there was energy that his body used to do work on his body to lift it off of the ground and over his wife's head. There are many different forms of energy and so we're going to take a look at those different forms and highlight the ones that in this course we're going to keep coming back to.