That was a question asked of an acquaintance of mine by her daughter, a reasonable query for a toddler in the months after the flood of 2005. The child's innocence blunted the bigotry inherent in the question and made it a pretty funny story that got passed around friends.
But it was, in truth, a darned good question, worthy of a straight answer.
So, little girl, why do Mexicans live on the roof?
Short answer, so that we can live under one tomorrow.