Have you ever been really stuck when your pre-schooler asks you a question (usually in the supermarket queue when everyone else is listening)?
The basic rule is to be honest about what you know and don't know. If you haven't got a clue, say, "What a very interesting question. Hmmm - I'm going to have to look it up" - which at least gives you enough time to Google it.
Remember, too, that the odd bit of truth-stretching is OK. If this makes you feel guilty, get hold of a copy of Andrew Riley's Great Lies to Tell Small Kids, Hodder & Stoughton, £8.99.
Why are you drinking wine, Mummy? Because wine makes Mummy clever...