Cell phone costs

When my son was about 5, we were riding in the car and I noticed he had my cell phone, examining it rather closely. Now this was some 15 years ago, when you were charged for both incoming and outgoing minutes on cell phones. He was fascinated with it, but I had always told him we only use it when we need to because it costs money every time you use it. So after studying it for a while, he asked, "So, where does the money go?" "What?", I asked. He replied, "You said it costs money. Where do you put it in the phone?" He was looking for a slot to insert your money! Funny to me, but simply logical to him!