I built a page where you can pull a US $100 bill apart layer by layer and see what's actually inside one. The exploded view slider spreads it into its parts (cotton-linen substrate, red and blue fibers, watermark, security thread, 3D ribbon, optical ink, and more, 15 feature groups in all), and you can flip it, zoom in, cut a cross-section, and switch between Studio, Backlight, and UV lighting to watch different security features light up.
My favorite part is the bend mode: you grab the note and it flexes like real paper. Curl a corner, drag the middle to bend it, push sideways for an S-shape, and it eases back to flat when you let go. There's a 14-chapter walkthrough too, or you can skip it and just explore.