@Knittinggrace
They want very specific answers to: Why are you here? Who will you see? How do you know them? Where will you stay? How long will you be here?
Prove everything with documents like names, addresses, hotel reservations. Your mobile may not work, so have screenshots or printed papers.
In the end it turns out they were concerned that I was going to work in Canada. I had my boots and hard hat in the back seat, because I drove there right after work. 5 officers will search your car and will lie to you to get you flustered. Be cool.