Why do kids lie?

This has been a worry for me for many years.. I know that my kids lied to me.. hid things and so on.. I'm over that now.. and they have all turned out so great.. Now I have one boy at home.. for the most part he is really good.. he's a good kid.. but then I find out he's lying about things.. nothing big.. but it still bothers me.. did you clean your bathroom.. yes.. do you have soap and shampoo.. yes.. did you wash your hands after taking the dog out and then before making her food .. yes.. All of those were lies.. Why?  When it would be just easier to tell the truth?  He has to know I'm going to find out.. and then what?  I'm the full time stepmom.. I don't want to be mean but this stuff really bothers me..