When I went over the SSM bridge, it was an easy explanation: "Why are you going to Canada?" 'To blow off some time between now and my Nexus interview, and I've never been to Canada and she wants to devirginize her passport *points over to passanger*' On the way back... "What did you do in Canada?" 'Blew off some time before my 1:30PM Nexus interview at the CBP hall over there and devirginized her passport *points to her*'
Both times waved thru quickly.