What is your most important tool?

I don't just mean a wrench, unless thats it for you. I mean any tool you regularly use for work that you consider your most important. If you're a software person name whatever digital tool you guys use. Whatever the career everyone uses tools I dont care if they are digital, verbal, physical or psychological. For me its honestly probably a flashlight, before I use any tools I'm always investigating with my flashlight or using a laser to trace lines.