One day, e^x sees x^2 running down the street in a panic. "What's wrong?" asks e^x. "There's a Differential Operator in town!" yells x^2. "If I run into him too many times, I'll disappear!" "Don't worry," responds e^x. "I'll go have a chat with him. No, don't worry about me -- he can't hurt me. After all, I'm e^x." So e^x walks down the street to the Differential Operator. "My friend tells me you're a Differential Operator," e^x says pompously. "Well, I'm e^x." "Pleased to meet you, e^x," says the Differential Operator. "I'm d/dt."