What's OP_CAT and why do I care about it? Bitcoin's computer is a bit like a clunky scientific calculator, the kind used in high school. Each button is called an "op_code." And from time to time, the Bitcoin community may agree on adding a new button. OP_CAT is one such potential button. Satoshi added it, then removed it, back in the day. What OP_CAT does is something pretty simple: it joins together two strings of characters, like "copy-paste", so it's a pretty simple and safe button. But if you add it to the scientific calculator that is Bitcoin, suddenly you allow Bitcoin's computer to verify ZK-STARKs and other proof systems. It opens the way to rollups and massive scale, in a self-custodial way that stays true to Bitcoin's and Satoshi's principles. I'm 100% sure Satoshi isn't around. If s/he were, s/he’d have had OP_CAT (and many other op_codes) on Bitcoin long, long ago.