How to get a housemate to do housework??

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And pick up after herself?? I have a housemate who is only 18 but with the way she treats the house & her excuses, you would think she is much younger. I have spoken to her in the past that if something needs doing, then just do it - she's not home much but she seemed to understand & agreed but she STILL doesn't do it. Take yesterdy for example, she came home for 5 mins with her boyfriend & a friend & the 3 of them had a drink then left- she couldn't be bothered putting the dirty glasses in the dishwasher!! Also a couple of weeks ago, she started spending more time at home so I left mopping the floors coz she has to pitch in too - well, 2 weeks later, they were STILL grotty so I ended up mopping them as I don't like living in a pig sty. Her excuses are because she cleans at work (she works part time in a bakery) that she doesn't feel like doing it when she gets home - I think boo hoo, we ALL have to do stuff we don't feel like doing...what about professional cleaners?? do they want to clean their own house on their day off?? I don't think so!! Any ideas friends?