If You Could Take Just One Item Back In Time, What Would It Be?

You're about to go back in time, but you don't know when and you are allowed to bring just one item with you. What item do you choose and why?

You could be going back in time five decades, five centuries, or five millennia. What modern item—a book, a vehicle, a gadget, a weapon—do you take with you? Something that would protect you? Something to impress the people of a past society? Something that could change the course of history? Tell us your pick and explain why you chose that particular item.

Sorry, no sonic screwdrivers allowed. This has to be something you could actually make or obtain in our present.

Top image: Ash brandishing his boomstick in Army of Darkness. Its utility might be limited, but it's good for scaring the locals while you still have ammo.