I think I stopped believing in Santa when I was about... 6 years old (after going to school, that is). My mum told me she had been the one putting all those sweeties in my sock. At first I didn't believe her because my cousin had told me about seeing an elf once (who would you trust, your mum or your best friend, eh)... She was a couple of years older than me though - suppose she just lied.
But I do have my own personal Santa who shares her candy with me during Christmas. So Santa's spirit isn't quite dead yet.
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