Engineer explains why the other checkout line always moves faster, does nothing to help

Just in time for last-minute Christmas shopping, here’s an engineer explaining why you’re most likely gonna get totally screwed when it comes time to pay for those pine-tree air fresheners and tube socks you’re buying at 9:30 p.m. on Christmas Eve. He doesn’t explain how to actually pick the fastest-moving line, mind you; just why every other line appears to be moving faster than yours. (Spoiler: because it probably is.) So, uh, happy holidays?