In the clip, a little girl is telling a joke. It's the kind of joke that, if told by an adult, we wouldn't find funny.
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But for some reason, the little girl makes it seem hilarious.
She says to (who we're assuming to be) her mummy: "Knock, knock"
The mum of course replies: "Who's there?" It's definitely not her first time.
"Atch", says the little girl. "Atch who?" Asks mum. Cue giggling from us.
Atch-who, geddit?
Anyway, we dare you not to laugh – or at least smile – at this video.