A little over 15 minutes but it’s a great watch. For a topic that, in practice, is very heavy in rigorous mathematical formalisms, topology is presented here as a very natural sort of idea. 3Blue1Brown is a fantastic YouTuber as is, and I think this video really showcases his ability to get the viewer to the core idea without being too weighed down by the mathematical formalisms it takes to get there. He makes a 100% purely mathematical idea interesting to the layman. He also has series on Linear Algebra and Calculus that are both excellent for anyone who’s currently taking those courses, or has taken them. Like I described earlier, his main focus is to drive home the core idea of what’s going on, it’s much less about the mathematical formalities and algebra, and much more about what is actually going on.