When young children see others fail to achieve a goal, theyspontaneously help. But there are many reasons why someonemight fail, and consequently, many ways to help. In order tohelp effectively, we need to understand why someone is fail-ing, so we can address the cause. One important distinction iswhether the failure is due to the agent’s own actions or some-thing external to her in the world. Here we show that 24- to48-month-olds can use their past experience to reason aboutthe probable cause of another person’s failure and provide helpappropriate for that cause. Children’s help targeted the worldwhen their prior knowledge suggested that the source of fail-ure was external to the agent, and targeted the person’s actionswhen this source appeared to be internal to the agent.