The boys, who were cousins, had been working on a model rocket project in a home when something went wrong, CNN reports.
Seattle police believe they were trying to start a fire in a fireplace and used some of the rocket fuel to get it going when there was an explosion.
The blast was so powerful, it busted windows in the home and blew open the back door.
Both boys were taken to the hospital in serious condition with fragmentation wounds, but they are expected to be OK.
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