Follow the prescription

Jack: "My brother was sick and went to the doctor."
John: "Is he feeling better now?"
Jack: "No, he has a broken arm."
John: "How did he break it?"
Jack: "Well, the doctor gave him a prescription and told him no matter what happened, to follow that prescription. And the prescription blew out of the window."
John: "How did he break his arm?"
Jack: "He fell out of the window trying to follow the prescription."