According to The Hitchhiker’s Guide to the Galaxy, what is the most useful thing an interstellar hitchhiker can have?

According to The Hitchhiker’s Guide to the Galaxy, what is the most useful thing an interstellar hitchhiker can have?
Moviestillsdb.com // Copyright by Touchstone Pictures and other relevant production studios and distributors.

The five-book trilogy of The Hitchhiker's Guide to the Galaxy is peak British humor (or should we say, humour), but also a genuinely good idea. Think about it: are you really going to buy your own spaceship to travel the galaxy or would you rather just stick out your thumb and catch a ride? I’m betting the cost of space-gas alone would be out of this world.

Space hitchhiking will no doubt come with a lot of risks. Trust me, you don’t want to get probed. On the plus side, there’s tons of cool sci-fi technology you could mess with. 

However, the books make it very clear that there’s one specific item any space hitchhiker absolutely needs with them before hitting the space-road. Thankfully you probably have one lying around, but do you know what you need to grab when the demolition ships show up?

Click START to answer.