When one is asked what it is exactly you do with your life and you are the kind of person who does not care to talk about yourself with people you do not give two figs for, you have just two options. If you find the question is asked by someone like, say, an irritatingly chatty hairdresser with whom you do not want to be excessively rude for fear of leaving with a diagonal fringe but can easily replace later with someone less perky, lying outright is the easiest and most amusing method of dealing with this annoyance. I have been a professional lacrosse player, a Christmas cracker riddle writer and a Russian diplomat in my time and never once caught the merest whiff of suspicion from the overly groomed teenager wielding the scissors, even when the appointment dragged on far longer than I had anticipated and my Russian accent had turn distinctly Swedish by the end. However if the person asking is, for example, an uncle whom you thought had been dead for a decade and your mother's stony glare over his shoulder is telling you silently but firmly that Santa wont be visiting any naughty little girls this year if they tell fibs, one is forced to admit to the truth and confess to being a PhD student.
This is invariably met with one of two responses. The first and mouthwateringly rarest is a genuine interest in your studies and topic of research, possibly followed by intelligent and easily answered questions about how you're finding it and how things are progressing. If you meet someone like this and they are not a fellow postgrad, you should marry them or at least see what they're like in bed at the first available opportunity. Rub yourself against them whenever you can; they're a rare breed and you'll feel their lack when they've gone. The second, and woefully much more common response, is to ask you what you're going to do when you've finished. This clumsy, sweeping assumption instantly dismisses all you're currently doing and the very concept of learning for learning's sake before it in one lazy gesture that dismisses academic achievement as merely a stepping stone to something else, probably involving more money. If this second response also includes the sneering little phrase 'real world' anywhere about its oily little person, you may just have to sell your soul for a snappy comeback and forgo the nuts in your stocking this year.
This is what I do.
30% paid employment
21% British Library
5% lying under the duvet complaining about not wanting to go to either of the above
6% watching porn, damply *
5% sitting bolt upright at 4am drenched in a cold sweat and gripped with a colon-twisting terror of someone publishing a thesis on the exact same obscure subject as me before I can get there first
10% eating leftover holiday snacks while reflecting that the most sensible reason for buying large quantities of food is not simply because it has an amusing foreign name
7% oooh look, such a lovely dress on Ebay, and so cheap too....let me just see if there's any more like it...
2.5% buying and neatly arranging new stationary. Matching folders and pencils simply vital to academic success
00.8% dancing in the aisle of Sainsburys after discovering the existence of Creme Egg bars
4.9% dancing in pants to Iggy Pop until spotted by every single one of the neighbour's eight children watching interestedly through an open window
00.2% suddenly and terrifyingly unexpectedly seeing supervisor on Channel 4 news out of the corner of the eye and throwing a mug of tea and a plate of curly fries over oneself in a desperate and hopeless attempt to turn the volume up
8.1% filling in continuous forms for an ever-inquisitive research committee on my progress so far without using the words nowhere, too, busy, filling and forms
*A Bit Of Fry & Laurie, series one

I can't stand that question "What do you do?" When I was unemployed, and was actually "doing" more than I do now, I used to reply "Well, I do lots of things, but I don't get paid for any of them."
Why was your supervisor on Channel 4 news?