Why Do Girls Wear Diapers?

Caitlin looked at me with all honesty the other day and said "Mommy, why do girls wear diapers?"
Of course, I laughed. Then I explained that it is just because she doesn't know how to use the potty all the time yet.
Conall laughed at her and started calling her a baby and telling her that it was because she is a baby. I explained that girls aren't the only ones who wear diapers and that Conall wore one until he learned how to use the potty all the time. Conall got really quiet and quit teasing his sister!