A girl's hilarious failure to solve this seemingly simple riddle was caught on camera by her family.
Her parents laugh uproariously at her repeated unsuccessful attempts to convince them of her own logic while sitting at their kitchen table at home in Warrington, Cheshire.
Clearly frustrated, the teenager earnestly explains her own reasoning for the riddle while her parents film their confused daughter.
Her parents ask her: "If a mother and a father have four daughters, they each have one brother, how many are in the family?"
The daughter replies emphatically: "10!"