The boy was attempting to set fire to his farts as part of a competition against his cousin in the garden of a house in Tipton when the accident occurred.
Fire fighters were called to the address but the small blaze had already burned itself out when the crew arrived.
Officers administered first aid until an ambulance arrived.
The victim was taken to Russells Hall Hospital in Dudley suffering from 18 per cent burns to the backs of his legs and his thumb.
Watch commander Paul Harpin, from Tipton station, said it was the first time he had been called out to deal with such an incident.
The boy had been pranking around in the garden having a competition with his cousin, when they were breaking wind and lighting it. Right behind him was a petrol can and that just flashed.
I think he must have won the competition but he will have some nasty burns now.
It is a warning not to mess around with fire, he added.
