What CS concept should be visualized next?

I launched VizLearn today. It’s a visual learning project for algorithms, AI, and computer science.

The thing I’m trying to get right is not just “show an animation”, but letting people change the input and step through what happens.

Right now it has lessons for sorting, data structures, MNIST, backpropagation, attention, scheduling, TCP, and a few more.

If there was one CS topic you always found too abstract, what would you want to see visualized?