 Stimulus Delta. Sorry. The only thing that keeps popping back into my head has to do with things I probably shouldn't talk about on YouTube. We don't have any sponsors. I guess it doesn't matter, but I don't really need to offend everybody in their brother right now. So no, a stimulus Delta, an S Delta, right? So Delta, it's a change. I suppose it looks like this normally. Anyway, so SD, no, no, not an SD, S Delta. It signals that no reinforcers are available for a particular response at this moment. It's a bit of a challenging concept to think about, right? But it says that, you know, that talking in a class is appropriate unless someone has said, please hold your commentary until the end of the lecture, or please hold your questions until the end of the lecture. Please hold your questions until the end of the lecture. That is your S Delta. It's telling you that if you ask a question, you are not going to receive a reinforcer for it. It might be an SDP. It might, you might be punished for it, but specifically it is an S Delta saying, hi, it is the professor will not reinforce you if you ask a question until I have authorized you to ask the question, right? So, example is a bit extreme, if you will, and kind of intense, but that's what an S Delta is. There's another absolutely great video demonstrating an S Delta that you should watch as to as part of the Behavior Beast series if you want to see him in action.