The phrase comes from Portal, Valve's 2007 first-person puzzle game built around a handheld portal device. 'Now you're thinking with portals' was used in its marketing and tutorials to encourage players to solve test chambers laterally rather than directly. As Portal became a cultural hit, the line spread to forums, videos and image macros as shorthand for clever, unconventional thinking. This truncated version leaves the last word implied, which makes the reference instantly recognisable to fans while still working as a reaction to any smart workaround.