Pen Malfunction

I took Mom to the grocery store today and at the checkout line, she wrote a check to pay the bill. The cashier asked her to write her phone number on the check. Mom asked me to look at the check to make sure that she had written it correctly. She wrote a zero in place of a four in her phone number. I told her her mistake and she said, "I wrote a zero instead of a four, because THIS PEN doesn't have anymore fours."