Every Hitch Hiker worth his salt has a towel. My towel looks like this:
The Hitch Hikers Guide to the Galaxy has a few things to say on the subject of towels.
A towel is about the most massively useful thing an interstellar hitch hiker can have. Partly it has great practicle value - you can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft down the slow heavy river Moth; wet it for use in hand-to-hand combat; wrap it around your head wo ward of the noxious fumes or avoid the gaze of the Ravenous BugBlatter Beast of Traal (a mind-bogglingly stupid animal, it assumes that if you can't sea it then it can't see you - daft as a brush, but very very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it seems clean enough. More importantly, a towel has immense psychological value. For some reason, if a strag (non-hitch hiker) discovers that a hitch hiker has his towel with him, he will automatically assume that he is also in possession of a toothbrush, face flannel, soap, tin of biscuits, flask, compass, map, ball of string, gnat spray, wet weather gear, space suit etc., etc. Furthermore, the strag will then happily lend the hitch hiker any of these dozen or so items that the hitch hiker might accidentally have 'lost'. What the strag will think is that any man who can hitch the length and breadth of the galaxy, rough it, slum it, struggle against terrible odds, win through, and still know where his towel is is clearly a man to be reckoned with. Hence the phrase which has passed into hitch hiking slang, as in 'Hey, you sass that hoopy ZarniWooP? There's a frood who really knows where his towel is.'