heh, I experienced that from the other side. I was 16 and my mum's best friend had just had a baby. It was her third, she had two older daughters aged over 10. It was the first day back from the hospital after her c-section, she was knackered, and I was already experienced and confident with babies, so I was left to feed and change the baby while she went outside with my mum for a coffee and a smoke and a relax. As I was beginning to change the baby, the door rang and one of the kids went to answer it. Someone came in and hovered in the doorway watching the baby, I assumed it was a relative and said "just a minute" while I deftly en-nappied and babygro-ified the sprog. Then I looked up to see a midwife/health visitor type person in full uniform, smiling at me and saying "you've got the hang of that quickly! And you're both looking healthy too! How's your scar doing?" I started to try and explain that I wasn't the mum, but the younger sibling got there first with "that's Mary! Mummy's out back having a coffee and a fa-a-ag!" The poor woman's face crumpled.