Just that, really. My mum loves anything magical/mystical, so was very heavily invested in us believing in Father Christmas/The Tooth Fairy/The Easter bunny when we were children. I actually tried to talk to her and tell her I didn’t think I believed in FC anymore when I was about 8 or 9 and she refused to hear it and told me that he’d be upset if he heard me saying that 
I did genuinely believe in Father Christmas as a child (until I was about 7 anyway) but I don’t think I ever believed in the tooth fairy. I found the idea of fairies wanting children’s teeth quite creepy and I found the idea that they switched them for human money strange. I think I would have found it more believable if they’d left a little gift instead of money...
I did put my teeth under my pillow to start with (there’s no way 6 year old me was turning down a nice shiny £2 coin), but I think I always knew it was my mum. I did ask her about it a couple of years later and she admitted that it was her. She obviously wasn’t as committed to being the tooth fairy as she was to being Father Christmas!
I was just wondering whether I was an especially grumpy and cynical child, or whether there are other children who might just be in it for the pocket money!