The Towel is perhaps the most important invention of whatever century it was invented in. The Towel is the most massively useful tool to take with you on your trips throughout the universe. It is handy for oh so many reasons: you can sleep on it, rub food and sauces on it for later consumption, use it to signal for help, wrap it around your head to avoid the gaze of the Ravenous Bugbladder Beast of Traal, or to dry off. And most importantly, strags (non-hitchhikers) will assume if you know where your towel is you are also in possession of quite a number of other common items like a toothbrush or a space suit (which means they are more apt to lend you said items if you ask to "borrow" them).
Douglas Noel Adams (11 March 1952 – 11 May 2001)