The average age a child stops believing in Santa is eight, according to a new study.
Researchers in the UK asked about 1200 adults from around the world about the magic of Christmas, and when it was lost for them.
They found a third of them wished they still believed in Santa, and two-thirds continued to pretend he was real well past the day they figured out he's not.
Fifteen percent said they felt betrayed by their parents when they found out, and 10 percent said they got angry.
Thirty-one percent however admitted lying to their own kids when asked whether Santa is real or not.
"It has been fascinating to hear why they started to believe he is fictional," said psych professor Chris Boyle of the University of Exeter.
"The main cause is either the accidental or deliberate actions of parents, but some children started to piece together the truth themselves as they became older."
One respondent said they found out when they caught their parents eating Santa's milk and cookies, and another when their father - drunk on Christmas tipple - noisily dropped a present.
Another realised she'd seen a present from 'Santa' sitting in her parents' room a few weeks earlier. One survey participant said they worked it out when they noticed Santa had the same handwriting as her father.
Some said they found out at school when their teacher told them. One person admitted punching a boy who told them the truth.
The Santa myth works, though - more than a third admitted they were on their best behaviour in the lead-up to Christmas Day, lest Santa fill their stockings with coal.
The study is ongoing, with more results expected to be published next year.