...when you realize that the other two year olds are still learning to identify numbers and to count; not doing addition and subtraction in their heads.

...when one of your twins teaches himself how to read at three years old, and you find out when he reads a book to you and your husband.

... when the other twin decides at age five that he wants to learn how to read and and insists that he will teach himself. AND HE DOES, within a couple of weeks, by practicing with BOB books for 5-10 minutes at night alone in his room before bed.