One of my favorite assignments for US history has been to have students research a member of their family who came to the US. As part of the assignment they had to say why the person came here which many could not figure out because the person came too long ago. But I made them research the time period and look at the originating country and the US and make some educated guesses just like any historian might do.
That is why this research from PEW is so interesting as it shows that most states' top immigrant population is from Mexico as opposed to Germany one hundred years ago. It also shows the top immigrant nations in the intervening years.