I'm sorry, but I do Santa, and it's lying. I'm pretending to my children that something is real, when I don't believe that it is. That's lying.
While I'm generally very, very honest with my kids (I've been accused of being too honest, particularly with ds1), and I don't like to think of myself as a liar...I am one. I'm lying. I maintain the charade all year, and I hide things from them, and tell them that a fat man is going to come down our chimney and eat his cookies and give the baby carrots to a reindeer. I know that's not going to happen. So...I'm lying. As bizarre as it may sound, I'm not dishonest enough to pretend otherwise.
As my mum said "Do what you like, just don't ask me to do it." And watch to vocab too.