We (I say we, I don't know who actually came up with it. Wasn't me, but I'm not exactly giving it away either) told my youngest brother that the reason the tooth fairy forgot to leave some money under his pillow for two (TWO!) nights in a row was because the earthquake in Japan shook a lot of people's teeth out and she was busy I'm actually shocked Mum's letting this one continue.
I also managed to convince him that my passionfruit halves (with all the goodness scooped out and eaten) was a geode, and he was rather panicked when Mum threw it away.
A friend of mine was convinced by her older sisters that if you put some ham in the VCR it would play a short film about pigs.