"Spreadsheets", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License
My brother once asked me if there was a function to produce a calendar grid from a list of dates in Google Sheets. I replied with a single-cell formula that took in a list of dates and outputted a calendar. It used SEQUENCE(), REGEXMATCH(), and a double-nested ARRAYFORMULA(), and it locked up the browser for 15 seconds every time it ran. I think he learned a lot about asking me things.