A woman dog walker leapt into action when she saw a group of girls set fire to a heap of hay in a Horsham field.
Police say that the woman grabbed a photo of the three teenage girls before running over and putting out the fire.
Officers say the woman was walking her pet in Chesworth Lane, Horsham, on September 2 when the incident happened.
The girls ran off after lighting the fire.
Police say inquiries are continuing into what happened.