When young children see others fail to achieve a goal, they spontaneously help. But there are many reasons why someone might fail, and consequently, many ways to help. In order to help effectively, we need to understand why someone is failing, so we can address the cause. One important distinction is whether the failure is due to the agent’s own actions or something external to her in the world. Here, we show that 24- to 48-month-olds can use their past experience to reason about the probable cause of another person’s failure and provide help appropriate for that cause. Children’s help targeted the world when their prior knowledge suggested that the source of failure was external to the agent, and targeted the person’s actions when this source appeared to be internal to the agent.