My dd (4) wanted to talk to the baby so she came and sat with me and started talking to my boob. When I asked what she was doing, she said she was talking to the baby, 'thats its head'. I had to explain (without laughing, this was very hard) that my boob was not the babys head!
Also, a little boy at her pre school went around telling everyone his mummy had a cat in her tummy!